My understanding is that in the activitypub fediverse there's considered a "true" version of a post/thread/etc. This is the one that is hosted on the instance that it was made. For example, the "true" version of beehaw communities is the one on beehaw. So all other instances then ask beehaw for the true version, and beehaw sends it to them. When someone makes a post on a beehaw community, they send that info to beehaw specifically. Then we simply indirectly see each other's comments due to syncing with beehaw.

When someone then gets blocked by beehaw, such as lemmyworld, lemmyworld still has their copy of the beehaw communities and threads. It's still their copy of it on lemmyworld. However, they can't go ask beehaw for updates, and they can't send their content to beehaw. As a result, no other instance would get lemmyworld's posts on beehaw communities, and lemmyworld will stop seeing new posts on those communities from other instances (other than beehaw).

There might be something that lets the instance grab the data from another "non-true" version. But I don't know if that's the case. I was told no, but idk.

I was kind of hoping that if an instance subscribed to another instance's community, then the originating instance can go down without effecting the community because another instance is now acting as the backup.

My understanding is no. While each instance makes a copy, I think without the originating instance, they end up falling out of sync. There is a way to fix this theoretically (since p2p does it), though I don't think the fediverse is doing this. I think it might work for mastodon posts that you "reshare" though? I'm new to the fediverse so idk the details.

The way I think of it is more like internet achive copying reddit posts. if reddit dies, IA still has those posts. Yet IA's version can't accept new posts, and if reddit dies they don't get anymore updates.