Explanation
Power BI uses Azure Active Directory (AAD) to authenticate users who sign in to the Power BI service, and in turn, uses the Power BI login credentials whenever a user attempts to access resources that require authentication. Users sign in to the Power BI service using the email address used to establish their Power BI account; Power BI uses that login email as the effective username, which is passed to resources whenever a user attempts to connect to data. The effective username is then mapped to a User Principal Name (UPN) and resolved to the associated Windows domain account, against which authentication is applied.
Reference: https://docs.microsoft.com/en-us/power-bi/enterprise/service-admin-power-bi-security