>>6
SSB is an experiment in distributed gossip networks, they aren't what one would classically consider a microblog network, but to my understanding they could do it. Their focus is more on enabling offline/mainly offline usage. This is also a real peer-to-peer network, while most ActivityPub implementations gear towards a decentralized model (more servers with fewer clients).
>>9
Consider understanding the networks and what they do, before studying their protocols.
Mastodon, or any ActivityPub server implementation doesn't necessarily distribute content amongst servers, and certainly doesn't rely on a central instance. AP is based on JSON-LD, that was created by the RDF people. That means they they heavily employ links and formal specification of their content. If you want to have a general idea of how it works, consider reading the introduction to the speicfication: https://www.w3.org/TR/activitypub/#Overview