I've just wrote one such mechanism myself.
Relays are setup by volunteers on virtual machines. Devices from users choose which relays to be reachable from (based on region or availability) and relays share the list of connected devices between themselves.
This way phones can connect quasi-P2P between them with a very light format that doesn't waste battery and doesn't require "clouds" for storage.
Very similar to NOSTR relays, the main difference are socks and a few extra services like file sharing and search.