Help understanding the mixer of a superhetrodyne receiver
U5A is a differential amplifier with a gain of 5. U5B is not "infinite gain". It is a 2-pole bandpass filter with a peak gain of about 15 dB at 128 kHz.

So the outputs of the mixer are filtered to remove any DC component by the first op amp, and the result is bandpass filtered at the expected modulation frequency. In the absence of a signal (zero amplitude) the output will be whatever noise has been detected at about 128 kHz. At ON amplitude, the output will reflect whatever the signal strength was. And since the filter is low Q (about 1) the rise and fall times of the output will be short.