In freely associating networks the topology emerges from the interplay between the constraints of the protocol and market forces
The Fediverse is a classic federated topology

Blue represents clients and orange represents servers ("instances" in Fediverse parlance)
Servers may choose not to federate to other servers
Users may self-host (one server per client) in which case the distinction between client and server is more a matter of code and computing distribution than a meaningful feature of the topology
Nostr is kind of half-federated with even greater fungibility between client and server

There is nothing stopping Nostr from rearranging into a Fediverse topology
There is nothing stopping Nostr or the Fediverse from rearranging into a centralized topology

Or even a distributed topology
