Relay (aggregates posts from all PDSs, responsible for federation of PDSs)
App views (generates feeds from the relay)
Bluesky hosts the only relay I'm aware of, because relays are very expensive to host. But afaict anyone can host a relay.
So if we're to describe it:
PDSs are nations states
Relays are the federation of those nation states
App views are like the ambassadors of the federation.
Except that PDSs can be part of multiple federations, and app views can be ambassadors for multiple federations. Ok not a great analogy.
So, non-federated means one company/organization has full control over some product/service/website (like Twitter, Reddit, etc).
Federated means that individuals can independently run instances of some product/service/website, and all those instances can talk to one another and share/sync data (Mastodon, Lemmy, etc).
Federation means that instead of one central server like Facebook or google, the service relies on several interoperable servers and you can pick which one you want to sign up with and use.
Email and lemmy are federated. If you want to use email, you can sign up for an account on one of many different servers run by other people or you can even host your own email server. No matter which you choose, you can always email another user regardless of where their account it.
The tech stuff is above my pay grade but essentially my understanding is the code is open source and one could start their own server on the platform theoretically but no one does
And yeah, a “federated” (but not in practice) Twitter clone