That’s not quite true afact.

There are 3 types of servers:

1. Personal data server (actually hosts your post)
2. Relay (aggregates posts from all PDSs, responsible for federation of PDSs)
3. 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.