@Doll_Tow_Jet@kbin.social I don't know if that's the official word for it but I'm using "backfilling" to refer to loading the history of a person or group after first federating with it.
the outbox is a special collection (list in activitypub speak) that's intended to work kinda like an old school email outbox where you put messages to be delivered and it would deliver them to the inboxes of people (and servers that were offline at that time would later pull them in from the outbox)
or, well, that's what the spec says. nobody uses it like that in reality because activity pub was intended for a completely different kind of social media than how it ended up being used (it seems to expect more "everything apps", including an entire client api that would completely abstract away the instance into nothing more than a dumb pipe for activities unlike the current reality where instance software dictate what you can do)
this probably confused you even more but it's getting pretty late here so I can't words good, sorry!
there actually is a way to "backfill" content, being the outbox. (although it was not meant to be used this way, from what i can see from reading what little documentation is out there)
lemmy uses it to federate about 10 or so (i think it was?) of the latest posts, and most microblog implementations use to federate the "pinned" posts of an account
activitypub has quite a lot of quirks, both spec-wise and implementation wise, and there are many reasons including an apparent near civil war inside the working group which resulted in this messy state it's ended up in
honestly the fact that this protocol works at all is a miracle