ELI5: How does Lemmy know to not show Mastodon posts?

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....