There’s a good comparison here.

Yeah, exactly. Let’s say you’re on instance B and I’m on instance C, and we both follow a community on instance A. You comment on a post, it will be stored on your instance B, and sent to instance A. Instance A will share it with my instance C because I follow the community, and it will be mirrored on all 3 instances. If instance A goes down, or doesn’t exist anymore, the old post/comments will still be there on instance B and C. If you leave new comments after instance A goes down, they won’t make it to my instance C. That’s my understanding at least, hope that makes sense!

As long as there’s a subscriber to the community on another instance, it will automatically have a copy there, that’s just how the federation works. It’s not really a backup, and it only has posts since someone subscribed to the community from that instance.

But if you’re worried about what will happen to old posts from lemmy.world communities: if you can read it now from your instance, it will still be there if lemmy.world disappears.

If you subscribe to a community in a different instance, it’s automatically mirrored there. If lemmy.world disappeared tomorrow, I’d still be able to view all the communities from lemmy.world on sh.itjust.works, for example.

If you post a new comment, it won’t leave your instance, but all the old posts/comments will still be there.