The TLD TL;DR is basically that domains don’t come out of nowhere. Just like how you need a lemmy.zip domain to be able to have the subdomains next.lemmy.zip or old.lemmy.zip, in order to have the domain lemmy.zip you must first have someone to run the .zip top-level domain (in this case, Google)
Like Forester mentioned in the other comment, you can have any combination of letters you want as a TLD, you just have to set up and manage all the infra for it (or find somebody else to do it for you)
As a matter of fact, if you look up a Lemmy community (or *bin magazine) on i.e. Mastodon, you’ll see it’s literally just a user that boosts all posts/comments posted to it
I don’t ActivityPub has any concept of communities, since even microblog-focused groups (like Guppe) work that way
Edit: not really, see replies