Theoretically there is community input but at the end of the day, one guy owns it and can do stuff unilaterally. There is occasionally drama over that, like recently the piracy community was removed (from lemmy.world, not from the whole fediverse) due to concern over possible legal hassles. I’d say Lemmy world tries to be friendly and mainstream, while edgier stuff tends to live on other instances. So you can still find what you want.

Nothing stops you from using multiple instances of course. It’s no different from drinking at more than one café. But people tend to have a main one that they visit the most.

Edit: clarify about piracy community.