They work like a handheld stud finder. You must hold the phone directly against a wall or the ground.
A legitimate metal detector only needs permission for your camera (to see the screen) and sensors. A fake APK will ask for: Satellite Nasa Metal Scan Apk App Download For Android
Users should approach such applications with a degree of healthy skepticism. Many "metal detector" apps for Android actually rely on the phone's built-in (magnetic sensor) rather than satellites. NASA - Apps on Google Play They work like a handheld stud finder