Vincent on Nostr: Let’s say the whole world goes nostr. There are billions of active npubs. I want to ...

Let’s say the whole world goes nostr. There are billions of active npubs. I want to find the outbox/inbox relays of someone. How does nostr go about this?
I cannot query all the possible 10002-kind relays (partly because it’s inefficient but mainly because I just don’t know them all).
DHT over pubkeys ? Are there other solutions?