This works because loggin in to outlook sets a cookie for login.microsoftonline.com which is reused by other Microsoft products. Each product requests a token from login.microsoftonline.com and as long as you have a valid cookie for that domain it will return a token that the app (outlook/teams/onedrive/etc) can use to access its own api.

The cookie itself stays bound to login.microsoftonline.com and it’s never shared with other domains and the service at login.microsoftonline.com will not provide tokens to requests from non-microsoft domains.