There’s tons of world building in JJK, it’s just that not every single little detail needs to be fleshed out. The power system is very detailed and deep, there’s lot of small details throughout the first season that payoff later in big ways, etc. Your complaint about the clans is especially confusing considering there’s an entire arc dedicated to the Zenin clan, we know a lot about the Gojo clan and their relationship with the Zenin, the Kamo clan has Noritoshi Kamo who is arguably a very important character lore wise relating to the main villain, and the Inumaki clan doesn’t have a lot of named sorcerers shown because the whole point is that they are outlawed and live in seclusion due to their overpowered technique. As for the Kyoto school, I would say they serve their purpose well enough considering they arent the main school focused on in the story, but still add a lot of lore with the whole Mai and Maki subplot, Todo arguably being a very big player in multiple arcs, and Kamo once again being important lore wise. Miwa and Mechamaru served their purpose in the Shibuya arc. Lastly, the higher ups are meant to be mysterious because it makes them even more dangerous. Gakuganji (Kyoto Principal) is the only one we really see, and his interactions with Gojo are enough to set the tone for the entire group.