This is a fun little fanon worldbuilding project that isn't meant to be taken seriously or as canon in any shape, size, form, or fashion. I will try not to break the manga canon (no promises though) but I will be bending it into a pretzel, especially regarding Uranus and Neptune. I don't see any reason this won't fit the anime as well but no promises there either.
Everyone had enough food, shelter, clothing, and all children went to school until 18. If someone was ill/injured they were able to get help for free. All of this was paid for by taxes and tariffs from outside trade. The Queen encouraged generosity among her nobles and the strong sense of duty shared by the majority of people ment most did not hoard their wealth (and were socially shunned to a certain extent) Nobles and Royals had a lot of stuff but most of it is heirlooms that had been slowly added to over generations. And while hard years were rare (due to magitech) when they did happen it was not uncommon for some of those heirlooms to be sold for charity purposes.
There was a fairly strict class system but there was some social mobility. If someone did something particularly notable or had powerful magic (or was gifted in other ways but this was rarer) they could even end up minor nobility. There was a bit of snobbishness about these new nobles but not too bad for the most part.
The Four Guardian senshi spent most of their time on the moon, but not all of it. They spent a month each year on their own world, each senshi a different month. The princess had never been off the moon (outside of sneaking to earth every chance she got) but she had been preparing to make a great year long journey to each planet (including the outers) when the Silver Millennium fell.
While planets did interact with each other directly, they preferred to come here for serious discussion. For the most part such interactions was between the planets in question, some things needed the Queen to be involved (royal marriages and the like).
All of the Noble houses across the Silver Millennium sent their heirs here, usually one year after they become adults, to spend a year to be formally introduced to the Moon Court. The year the heir became an a adult was spent at their kingdom's court. Other noble children also spent time here, especially if they or their parents wanted them to become diplomats.
Outside of being the hub of diplomacy the moon was known for magic. Many of the greatest magic wielders across the Silver Millennium and even beyond trained on the Moon at the Great Schools (yes several). The Four Guardian Senshi (aka the inners) trained at one of the schools some (to learn more general magic) but most of their training was actually done at the palace and in secret. Serenity also trained at one of the schools but mostly she was learning to use the Silver Crystal. This was done in secret with her Mother.
The Queen of the Moon had two counsels. The first one was to help her with her duties as the Moon Queen, mostly Lunar Nobles as well as a representatives from powerful groups such as some of the Mage Schools. The other was for all the planets of the Silver Millennium, this contained 1 representatives from each nation (except Earth ofc) and representative of large companies that did most of their work between nations (merchant guilds mostly). The Queen had the final say in the first counsel at all times, however, in the second counsel it was majority rule. With that said, her vote counted for two when not a tie and was the tie breaker if the vote is a tie. The princess had been allowed to join in these meetings right before the Silver Millennium fell but she didn't have a vote in any body (she was allowed to speak with the queen's permission though.)
All the people who came to the moon brought their culture to the Moon giving it a multicultural feel. With that said traditional Lunar culture was very playful and (usually to the surprise of outsiders) fairly informal most of the time. Parties were common both at the palace and across the city and (except for the palace of course) you didn't even have to be invited to go to one (although you could get thrown out for being a poor guest). They were also very fond of nature and spend much of their free time in the various gardens and parks across the city.
Most of the Castle was actually research labs and schools, as well as a massive library. The most technologically adept planet of the Silver Millennium, they invented most of the technology everyone used, as well as accepted students from across the System. Even the senshi and Serenity had classes via correspondence with them.
Their science was heavily influenced by the magic of their world, and it was very common for Mercurian students to either spend time at one of the Lunar Magic schools or used correspondence classes, and visa versa.
Mariner Castle's shield was able to both block solar radiation and, with the help of mirrors, concentrate it so that no matter if the castle was on the dark or light side they got only the radiation they needed.
Mercurians were a serious, hard working, group of people for the most part but swimming and other water sports were popular. They also liked chess and other games of strategy. Mercurians also loved beauty and their technology reflected this.
Mercury had an army but it was small, they did however invent most of the equipment for the other planets armies though.
Venusians tended to be artists, musicians and actors. Most of the great art, theater, and music came from Venus. Venusian fashion was the trend setter among the entire the Silver Millennium and even beyond.
Venusians were also a romantic people and all forms of love both romantic and platonic was celebrated.
While Mercury was usually known for their inventions the ability to make them able to handle Venusian weather was a local thing, and they guarded this secret very carefully.
Venusian warriors fought in small groups and the staff was traditional but they used other weapons as well. The army overall was neither the biggest or the smallest among the planets.
Martian's tended to be very martial, preferred to fight in large armies with the latest tech and were widely considered the best warriors of the Silver Millennium. In addition their military schools were the best, and while most planets trained their own soldiers, officers usually got at least some of their training here.
Mars was also the big manufacturing planet. Some farms did exist but they tended to be specialty items like spices.
Martians were possibly the most spiritual in general and it wasn't uncommon for older Martians, especially after leaving military service, to join one of the many religious orders on the planet
Io Castle was massive, the biggest in the Silver Millennium and was the main farm land. Besides areas around the palace there are several other floating farmlands, and farms on some of the moons. Most of these were protected with special shields against Jupiter's powerful radiation. Like with Venus this technology was developed locally and is fiercely protected.
Io itself also had mines where rare materials are found. Mostly Martians worked there though since Jovians hate being in areas with no greenery.
Jovian culture prized their family and community. They almost never traveled far from their homes and they tended to be a bit standoffish of strangers but if someone was willing to put in the work they made fantastic life long friends. They were hard workers but in their downtime they liked sports, crafting, gardening, and cooking.
They useed a militia type system where all adults between certain ages (with the exception of mothers of young children) trained for on alternate weekends. Because of this most planets tended to underestimate them but some Martians who had helped train them believed they could have given the Martian army a run for it's money.
Before I go to each planet I'd like to make a quick note on the Outer System and their Senshi as a whole
I tend to go back and forth between thinking these planets had no one on them, except Uranus and Neptune on their respective planets, and the below fanon. The first one however is boring for worldbuilding so, after some thought and careful reading of the manga (several translations) I decided to share this fanon. Besides, like I said in the top paragraph, I do not think this breaks canon but it does bend it pretty hard. *shrugs* But if I was the type to be slavishly devoted to canon I wouldn't spend so much time in the transformative fandom spaces.
Anyway, I decided that for Uranus and Neptune the senshi were kept very carefully in seclusion from most of their people and the people they did interact with were very formal and not especially friendly. I'd also note that the manga tends to view loneliness as being without someone who understands and shares your dream/responsibilities/powers, which would fit Uranus and Neptune since they were the only Senshi on their respective planets.
Pluto of course was at the time gates and before she went there I tend to believe she was also carefully kept in seclusion, mostly to prepare her for her duties as Sailor Pluto. And Saturn was on her planet as the princess but not the senshi, although she and everyone else on her planet as well as Queen Serenity knew she held Sailor Saturn asleep within her.
Most people did not know there was a population but there was. The population was very small, secretive, and it was actually illegal for any outsider (except for the Moon Queen and Princess and spouses of citizens) to step foot on their castle. Saturians had a separate station where they traded for their goods and they rarely left once adults. Older teens sometimes went to Mercury to finish their schooling, mostly to introduce them to the rest of the world, then all would spend a year on Pluto for their military service before returning to Saturn.
For a Saturnian to marry an outsider was a big deal and it was very rare. The potential spouse would have to give up all ties to family, for this reason most people are either orphans or were disowned, and home planet to become a Saturn citizen. this is the main reason the Queen traveled there, due to the strict nature a third party with no ties to either side was required to sign as witness. On the chance that someone of the Moon kingdom was wishing to join it would have been the Plutonian Queen, with special permission to visit, who would have been the witness. This had never happened though.
Another reason the Queen traveled there is for a once a half century State Visit to check on the people and make sure everything was ok.
Uranus had a large population, one of the biggest of the Silver Millennium. Uranus was the inner most line of defense against outsiders so it had a larger army than most would expect. Uranus was also where most out system merchants usually stop and trade so there is also a massive area for trade. While teleportation was preferred for transport, space ships were used, especially for large shipments, and those were also built and maintained on Uranus.
Foot and space-ship racing are both popular sports. With that said it was rare for an outlander to compete. Both because they knew they most likely wouldn't win and because Uranians were known for being cold and standoffish.
Neptune had large pools of water both for growing seafood but some were also used for swimming and other water sports.
Neptune was also a place for trade so they had a shipyard and areas where merchants could trade their goods. Their shipyard wasn't quite as big as Uranus's though, since while they maintained ships they didn't build them.
People liked coming to Neptune for vacation as well. As such Neptunian culture was somewhat more welcoming of outsiders than Saturn and Uranus. As long as those outsiders stayed out of any area marked off limits, anyway.
The actual population was very small, only Saturn was smaller, but they hosted some of the armies of other planets, so their population looked bigger than it actually was.
Diplomats had to sign in here before they went to the Moon or other planets. If the nation was new to the Silver Millennium the diplomats were especially carefully checked over before allowed further in.
Finally Plutonian and Mercurian scientists had a joint long term project where they studied the outside of the Solar System.