I found the factory test mode. It can be used to test all functions to determine whether the cell phone is working or not.
You can test eg Screen, Backlight, Touch, Sensors, Flashlight, camera... And more
!!!! But be careful there is an emergency call test !!!!!!
To get there you go into the settings, about the phone and there click on the Android 9 button then a popup window appears.
If you now click on the build number you get a toast message that you should click 3 times quickly.
Then the factory tool opens. It is all in Chinese but if you go