How does my iPhone know which headphone cable I plug in? When I plug in my Apple EarPod earphones, my phone sets the volume to whatever it was the last time, but when I plug in the auxiliary cable in my car (just a simple jack, no fancy infotainment system) it puts it to full volume, like the last time I plugged in that cable. Is there something special about the Apple EarPod cable? Or does the iPhone do something even cooler?
An line-level input, which is what your car's aux in jack is, presents a relatively high impedance to the source - generally 10k ohms. Your 'phones have a much lower impedance, typically 32 ohms for PMP phones. It's possible to distinguish between them in that way.