I now have an ESO account?

[Uther]

And the ID starts with 'A' + seven or eight numbers?

I don't remember opening this account. I don't know what validation ESO might have received or what email might have been used to open it. I've received nothing.

Why is this mechanism so opaque?

[Andrey]

You can login on eso website "through steam" and look on your game account - registration date, email.

Players create game accounts by themselves on website or maybe during first time they launch ESO (not sure about second). And create ID (login) in the process - no, it doesnt have to be A..numbers - any login.

ED: start ESO in your steam library and in the game launcher click on "Account" button - game website will be opened on login page.

[Uther]

Thank you.