Many smartphones contain miniaturized microelectromechanical systems (MEMS) magnetometers which are used to detect magnetic field strength and are used as compasses. The iPhone 3GS has a magnetometer, a magnetoresistive permalloy sensor, the AN-203 produced by Honeywell. In 2009, the price of three-axis magnetometers dipped below US$1 per device and dropped rapidly. The use of a three-axis device means that it is not sensitive to the way it is held in orientation or elevation. Hall effect devices are also popular.
They don’t. There’s typically a compass in phones that provides information useful in determining direction.
https://en.wikipedia.org/wiki/Magnetometer
Oh interesting. I guess it makes sense. Much simpler solution with high accuracy.