How does the iPhone calculate battery percentage and with what parts?

you'd have to talk to someone with more electronics knowledge, as its to do with the battery cells themselves... the percentage and estimation is done by a chip on the logic board, usually referred to as the SMC (systems management controller) or PMU (power management controller), but it doesn't actually know what capacity the battery has, which is why when the battery cells begin to die off, the percentage drains quicker than usual. its an estimate based on what the battery is currently holding and what the phone is doing to see how long it will drain based on its current usage...

On a computer it is much more comprehensive, as it allows you to see not only the current charge capacity in mAh (miliamps per hour), but also what capacity the battery would have had originally (Volts x Watts = Amps/1000 =mAh)) but the iPhones system is not able to tell these details... but the cells will react differently based on the temperature, as cold weather will slow down the lithium cells... but thats the extent of my knowledge. sorry