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 even 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, and when they are they are delivered to the users’ inboxes (although if public they can be read from a user’s outbox).