It works in many ways like AM radio. In this case since the return signal comes from many objects this blurs the return signal so I also suspect they use digital microwaves so a computer can analyze the return signal and make sense of it. Since microwaves also cook human flesh I suspect the noises people occasionally hear is a result of this digital heating effect.
The microwaves explanation seems plausible. I remember reading somewhere about a Soviet listening device that involved aiming a laser at an embassy window. The sound inside would vibrate the glass slightly and modulate the reflected beam and somehow they could extract out the audio signal. I’m not sure if they were found to be using it or it was just rumored to exist but it was sort of a similar principle.