The USB-C cable works both ways for me, with the S9 ProI believe any standard USB-C to USB-C cable works with the S9 Pro and the S9, though ironically the cable supplied with both devices is non-standard and only works one-way round. You do need an appropriate cable for connecting to the lightning port on an iPhone (or use the Camera Connection kit). Personally I’ve found the non-standard cable to be unreliable though qutie possibly I forget and plug it in the wrong way round quite often. I stick with a standard cable as used for any other device which connects to my iPad pro.