I have about a dozen email aliases associated with my Microsoft account. On the "Your info" page, under "Account info", one of them is described as "The email address you use to sign in to your Microsoft Account".

However, I can use any of them to initiate a login attempt. I have my account set to passwordless, I don't know if that is relevant (every login attempt triggers an MFA prompt).

If I click on "Edit account info" I am taken to a page where I can choose which address in the "Primary", but given that ANY of the aliases can be used to intiate a sign-in, I don't see any benefit in changing that.

EDIT: I wasn't being adventurous enough. The option to change which aliases can be used to sign in is under (surprisingly) "Sign-in preferences".

In my defence, that page wasn't loading properly in Firefox with all my privacy add-ons enabled. I was able to access it in Edge.

EDIT2: I've changed my primary alias to a newly created one. If I am still able to sign in OK in a couple of days, I will disable the old primary for sign-in. I hope I don't live to regret this!