Fediverse

As a new user to all these sites, I’m confused about they interact with each other. For example, on Lemmy, I can view a Mastodon account, but I can’t view that account’s posts. This makes sense, as sorting your home feed by “all” could hypothetically result in your feed getting flooded by popular Mastodon posts.

On the other hand, I understand that Mastodon users can see, reply to Lemmy threads, and subscribe to Lemmy communities. This flow of content makes far more sense than if Lemmy allowed Mastodon posts to be visible.

With all that said, how does Lemmy know to not show Mastodon posts? How does ActivityPub even work? How does ActivityPub understand that certain posts are from certain sites? Aren’t all these sites just different UIs that advertise different servers? I’m not gifted with technical knowledge, but I’m dying to understand how this whole thing works.