"Sign in is easy, convenient, and secure when you use Microsoft Authenticator. Use your phone, not your password, to log into your Microsoft account. You simply have to enter your username, then approve the notification sent to your phone. Your fingerprint, face ID, or PIN will provide a second layer of security in this two step verification process. After, you’ll have access to all your Microsoft products and services, such as Outlook, OneDrive, Office, and more."
Microsoft Authenticator on the App Store (iOS)
Get Microsoft Authenticator - Microsoft Store