How do Android EMF detectors work? Searching the play store for emf detector shows a bunch of apps that can measure the electromagnetic radiation level and they do seem to somehow work.

What is the operating principle behind these apps and what kind of sensors are they using? 

What level of accuracy these sensors provide and how do they compare to dedicated EMF detectors which sell online starting with ~$7?

In the Android docs I see: SENSOR_TYPE_MAGNETOMETER, SENSOR_TYPE_MAGNETIC_FIELD and SENSOR_TYPE_MAGNETIC_FIELD_UNCALIBRATED — can these possibly be used to measure EMF radiation levels and if so then which ranges are supported?

Update:


This question suggests that Android phones might be able to measure magnetic fields: https://skeptics.stackexchange.com/questions/6961/can-smartphone-apps-detect-emf
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.