> the exposure feature in my iPhone camera…choose the "middle gray" point of the picture for the tone mapping process

No, it uses that to set the physical exposure via the shutter speed and ISO (iPhones have a fixed aperture, so that cannot be changed). It literally says this in the video you linked. This is not tone mapping. Tone mapping in a way may also happen afterwards to convert from the wider dynamic range of the sensor if the output format has a more limited dynamic range.