When you are using the mobile app, there is not a log out button to push. You can exit the app by going to a different app or your home screen. Each time you navigate away from the app, your active session is ended, which serves the function of a log out button. When you navigate back to the app, you will be asked to re-authenticate, and if you have quick access features such as FaceID, TouchID, or passcode log on, you can gain access quickly each time.
If you want to remove the connection between your device and your account altogether, you can go to the Menu > Settings > Remove Profile. This will remove all association between your account and the mobile device and app you are using. Each time you attempt to access the app after this, you will have to put in the full username, password, and 2-Factor authentication to authorize your account on that device.