I recently found out by chance that you can enable Face ID on your mobile apps in iPhone. All you have to do is 'tap and hold' the mobile app icon until a window pops up with a list of options; tap 'Require Face ID' and tap 'Require Face ID' again at the prompt. The next time you try to open the app it will scan your Face before the app can be opened. If Face ID fails because you have not paid attention to the camera simply tap the display 'Try Face ID Again'. If later you want to disable it; simply repeat the first step and tap 'Don't Require Face ID' and look at the camera (Face ID is required to disable it).
This can be an added layer of security for a mobile app; as one can't even open the app without Face ID.