There is an already defined NIP for this, any app can just follow it to be interoperable. This way, People using nostr:npub1yjxerh4msvgqf230ej3648a28xhvxjstqr2rhpjt2eelf538e70scnl9d0 or Bitchat (or any other future app) will be able to sync and communicate https://github.com/nostr-protocol/nips/blob/master/BE.md