Explain "Audio Loopback Dongle"
I'll re-draw the circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

R1 and R2 mix the Left and Right signals together, and with R3 to ground provide some attenuation.

C1 couples the mixed input signals into the attenuator consisting of R5 and R4 - the junction of R5 and R4 is the output to the microphone input.

I assume that R4 is required in part to indicate to the computer that a microphone is connected.