Think of it like email (lists). There can be a !fuckcars@lemmy.world and a !fuckcars@lemmy.ml

You can access both communities, subscribe to both and post to both. Their content is (mostly) identical, the only difference is who’s hosting it.

There is no central authority determining the rules. For instance, Reddit can ban whatever they like and allow whatever they like. That’s not how it works here. The only rules are what each community decides are their own rules. Certain communities, such as !piracy@lemmy.dbzero.com no longer exist in some sort of tolerated limbo, unlike on Reddit where they could be shut down at a moment’s notice.