How do Android EMF detectors work?
The question does not only apply to android phones, but to all mobile devices. Our mobile devices use EMF to communicate. Mobile phones are, by definition, devices that can detect EMF since their ability to communicate depends on radiofrequency fluctuations in electromagnetic fields.  Our devices, especially smartphones, which are extremely reliant on internet services emit a huge amout of EMF radiation. That should imply that mobiles can precisely detect and manipulate EMF, right? No. Our phones are designed to work with only specific frequencies, from about 800 to 2.2GHz, so it is quite unlikely that a mobile phone can detect general fluctuations in EMF.
Now, the creator of the app might tell you that it uses magnetic sensor, and you might think that EMF can be detected using a magnetic sensor, since it is also (partly) a magnetic wave, but you cannot. EMF exists when a wave has both electric as well as magnetic parts.