I had this issue with a React app I inherited, there was a .env with credentials, and I couldn't figure out whether it was being read from the frontend or the backend.
So I ran a static analysis (grep) on the apk generated and
points light at face dramatically
the credentials were inside the frontend!
Why would you have anything for the backend in an APK? Wouldnt that be an app, that by definition runs on the client?
Most frameworks also by default block ALL environment variables on the client side unless the name is preceded by something specific, like NEXT_PUBLIC_*