Have a look at this page: https://docs.mendix.com/appstore/modules/oidc/ it should help you implement microsoft SSO sign in for your application
Yes, when users login to outlook they really login at Entra ID. Outlook delegates login to Entra ID.
If you delegate login to Entra ID as well (using OIDC SSO as Mathijs indicated), your endusers will get a Single Sign-On experience:
they login with same credentials as they use when doing a login to Outlook.