[KoftaBob]

> IIUC, you have to know which relay(s) have what you're looking for, and if you don't, you just have to guess.

This problem is tackled by an improvement to the protocol that was recently introduced called "NIP-65": https://github.com/nostr-protocol/nips/blob/master/65.md

The TLDR is that when a Nostr client supports NIP-65, it broadcasts to all known relays (which is continually updated/expanded) the list of relays that User A posts their stuff to.

This means that as long as User B is connected to at least one of those "all known relays", their client now knows what relays User A posts their stuff to, and will specifically fetch things from those relays when it needs to load User A's things.

It's essentially the Nostr take on the Gossip protocol: https://en.wikipedia.org/wiki/Gossip_protocol