I think the misunderstanding comes from where you wrote “it would create a new mail account” which is objectively inaccurate, the @outlook.com / @hotmail.com emails already exist as both a “mail” and Microsoft account - there’s literally nothing being “created” in that situation, you’re just signing in to what already exists.
In your father’s case, he probably has a Microsoft Account set up with a third party email address.
If he were to want that to include a mailbox, he could navigate to his Microsoft account’s email settings, create a new “@outlook.com” alias, and set it as the primary alias for the account. He would then have a mailbox usable at Outlook.com or via Microsoft Exchange in a mail client.
It’s possible to do the same thing with Google - you can create your Google account using a third-party email address, you won’t have a mailbox but if you were to visit gmail.com you would be offered the option to create a mailbox with a new @gmail.com address.
I think the misunderstanding comes from where you wrote “it would create a new mail account” which is objectively inaccurate, the @outlook.com / @hotmail.com emails already exist as both a “mail” and Microsoft account - there’s literally nothing being “created” in that situation, you’re just signing in to what already exists.
In your father’s case, he probably has a Microsoft Account set up with a third party email address. If he were to want that to include a mailbox, he could navigate to his Microsoft account’s email settings, create a new “@outlook.com” alias, and set it as the primary alias for the account. He would then have a mailbox usable at Outlook.com or via Microsoft Exchange in a mail client.
It’s possible to do the same thing with Google - you can create your Google account using a third-party email address, you won’t have a mailbox but if you were to visit gmail.com you would be offered the option to create a mailbox with a new @gmail.com address.