The manner the extent up/down buttons work is that one of the signals is shorted to floor so, for example the usage of the "android" (omtp) pinout, in case you press "vol up" then pin a and pin d is shorted. However, on an "apple" product, pin d is the mic and not anything takes place. This is glaringly definitely simplified. In actuality, there is maximum in all likelihood a change in resistance exclusive that what is already there from the left/right speaker or mic. Notice the switches. Let's use sw3 (ff/next) for instance. While sw3 is open, the microphone is connected and completes the circuit (green hint). When sw3 is closed (as in pressed), it shorts the circuit earlier than getting to the microphone bypassing it, it goes via a 600ohm resistance and completes the circuit at floor (yellow trace). The smartphone then sees the 600ohm resistance as a sign to "bypass" passes that to ios and ios skips to the subsequent song.