About the combination of two signals with capacitors
The capacitors just act as relatively high impedances to sum the two oscillator signals at D4.

D4 then provides the non-linearity to cause mixing (multiplication) of the two signals and create sum and difference signals in the frequency domain.

In this case the two oscillators are running at approximately 250kHz.  So the sum frequency will be >500kHz and the difference will be in the audio range.  The audio frequency is filtered by C23 and associated resistors and then amplifier to create the audio output.