Surface Duo is powered by the Google Android operating system, and allows you to sign in using a Google account (@gmail.com) and a Microsoft account (such as @outlook.com or @hotmail.com). You are not required to sign in with either a Google or Microsoft account, but if you do, additional options and features will be available.
When you sign in to your Surface Duo with a Google account, you also sign in to the Google apps that are installed on your Surface Duo device.
When you sign in to your Surface Duo with a Microsoft account, the Microsoft apps on your Surface Duo are integrated with OneDrive to keep your files backed up, protected, synced, and accessible on all your devices. Keep in mind that signing in with your Microsoft account does not automatically back up your Surface Duo photos and videos to OneDrive.