When a remote group is first discovered by local user, their instance creates a local space for that group and creates a link to it. On kbin, I think that’s all it cutrently does, though on Lemmy sites they also pull in a few recent posts.

Once a user actually follows a group, though, their instance subscribes to it as if it were an actual magazine subscription. New content gets sent to it, and shared among anyone who follows it, but old content doesn’t, any more than National Geographic sends new subscribers their whole 150 years of back issues.

Older content can get pushed out to newer subscribers if people boost those older posts, though. That would be the equivalent of the publishing world’s re-issuing of an old edition.