Same way. A majority generally have a old phone that was already signed in to google. Or if they remember only Apple ID and password (one very difficult password) + sms. They can login on to a new phone.

Everything is SYNCED immediately.

What if you have a ransomeware that destroy everything on the same day your house and all backups burn down. And you cant get it from immutable backups as you wrote that decryption key in paper. And the bank will not allow you to access it as the govt deported you elsewhere.