frequency modulation and incoming signal selection
First of all, realize that all forms of modulation add frequencies to that of the carrier.  These frequencies appear close to the carrier so that the modulated signal can be selected with a bandpass filter.  You do this by tuning the radio to a specific station. Then the signal from the selected station is demodulated. For amplitude modulation, a detector sensitive to amplitude variations is used.  For frequency modulation, a detector sensitive to frequency variations is used. This is a greatly simplified explanation.  For details, you need to look in any communications textbook or search the internet.