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When Mastodon federates, does it do so by polling all the different accounts I follow to ask for their latest posts, then save those results to its local Postgres? Does it also fetch images, etc?

Does it cache that ahead of time or when I show up?

Tell me what I should read—I'm finding the basics of ActivityPub okay to grasp, because I am familiar with their ancestors, but implementation details are confusing. Like what are the basic responsibilities of a "server" in a fediverse app? I know there's a post out there for me to read somewhere...