If you are having issues relating to custom token error when logging into mobile app. Make sure the following:
Same Firebase project for the mobile apps and the backend. Check how to setup firebase on mobile here: https://bakoambrose.gitbook.io/fuodz/flutter-app/firebase and how to setup backend firebase here: https://bakoambrose.gitbook.io/fuodz/backend/firebase
Make sure you have uploaded google-service.json to the backend. Steps are recorded here https://bakoambrose.gitbook.io/fuodz/backend/firebase