I was struggling to wrap my head around how federated social media works until I realized that email has basically been doing the same thing for 30 years. Different email servers are like instances of a federated network. You can send emails to people from within a single server or you can send emails to people on any other mail server. Your email address is a username followed by an ‘@’ and the server address, just like on Lemmy. Email is a decentralized service I’ve been using the whole time!
It’s not so much of an analogy, email actually is a federated technology just like ActivityPub is and ActivityPub works a lot like email and has audience targeting fields which map onto the same audience targeting fields that email has (
to,cc, etc.). Activities aren’t always publicly available, although they can be and when sent to specific people, they are delivered to another users inbox (although if public they can be read from a user’s outbox).