The Alby Mobile Extension is currently only available for Firefox mobile on Android. We are waiting for Apple to fix an issue until we can publish it on iOS, too.
The Alby Extension is independent of Alby in the way that it is a fully client-side app. You can run it even when Alby should not be there. 
But as  @rabble said you can use the same Nostr private key in many other apps at the same time.