[Y_Y]

I don't know what you mean here, I can plug a speaker into my mic slot and use that to record, just as plugging a mic into the speaker slot gives a (crappy) speaker.

[cdaringe]

> purposely built input stage

You moved your device to the purposely built input stage.

Not an expert, but your remark doesn’t compute with the parent comment

[Workaccount2]

Because on your computer the engineers purposely put a switch that can direct the signal to either the input hardware or the output hardware.

It's not a mic slot, it's a general analog I/O port with a 3.5mm form factor.

[15155]

A DAC and amplifier circuit is electrically incapable of processing input (on its own.)

Physically unplugging and moving a speaker to a mic input works, sure, but very few devices can do this switching electronically.

[tosti]

I think most intel HDA compatible chips can do this. You can specify which pins are connected to what.