Lemmy communities work a bit weird when (de)federation gets involved.

By default, any community any server member is subscribed to gets mirrored to your home server. If your home server defederates/gets defederated, that local copy is still available. Local users can still post content, post comments, and have all kinds of interactions, the rest of the Fediverse just doesn’t know about it. It’s a bit like a “fork” in git/blockchain tech.

I don’t think you can subscribe to the local copy of a different server (although perhaps technically you could implement that relatively easily, I think?) but in a way any remote community you follow is “cross instance”.

Lemmy doesn’t implement supercommunities like Reddit does with subreddits (you can’t merge the “technology” communities of different servers under a single name), though that feature has been requested a bunch of times. That would be a solution where rather than following a bunch of different communities on different servers, you could merge a bunch of remote communities together under one single supercommunity that everybody can then follow, allowing for an intricate network of local communities that’s harder to break up (until, of course, said supercommunity starts removing subcommunities for whatever reason).