If your device has biometric authentication (Android) or Touch and Face ID (iOS) set up when you first register the mobile app, you’ll automatically be asked if you want to use this to log in.
If your app is already registered, you can set this up by going to Settings > Biometric login (Android) or Settings > Touch ID or Face ID (iOS) and toggling the button to enable it. To disable biometrics, just use that same button to turn it off again.