FM Receiver, how it works?
There are several ways to detect FM.

Today, this is generally done with a PLL (phase-locked loop).  It's a voltage-controlled oscillator in a feedback loop.  Via feedback, the voltage into the voltage-controlled oscillator is adjusted so that its output matches the input signal.  This control voltage is the FM-demodulated signal.

In past days, other techniques were used, like ratio detectors.  If the carrier is slow enough, you can use digital techniques to outright measure the time between zero crossings, or the number of zero crossings in a particular interval.  Think of a low-pass and high-pass filter adjusted so that they both attenuate the carrier frequency equally.  Any change in the carrier frequency will cause a relative difference in the amplitudes.  There are more methods.  Look up "FM demodulation".