It may not sound like a big distinction, but there is a big distinction there. Instead of your username being your email address, they've abstracted the authentication from the core user record such that usernames are not used for authentication, so don't have to mutate if the authentication identifier (email address, phone number) changes, just some auth settings. This is obviously a much more extensible and robust way to deal with authentication over time, but it's also obvious it's much more complex than a simple username/password pair.

Re: implementation - I don't see the big distinction. These are all just related properties of a master user id number. Those properties can be anything and any of them (in any combination) can be used to login.

Nope. Most free email providers out there (including gmail) have policies for username expiration, some even quite aggressively (like one month without login). "Me" on one of these services might not be "me" in three months' time, but you-website won't know that and will happily send a password reset link to my old email.

Really, they do.