The instance is lemmy.world, the community is c/fediverse, while you can post whatever regardless of your instance, you can’t post whatever you like into a community, you need to follow the rules and themes.

Communities are after all the equivalent of subreddits while instances are just serverd where accounts reside and communities are hosted.

I would wager most of nowadays instances have either fallen into obscurity or just finished existing, I think we will see instancea more focused in scalability if thr fediverse grows in popularity, whoch will kind of dominate the space.