2 points
Depends on the implementation. Some indeed have it like you described, using some of the sensors used by ABS. others use tiny pressure sensors mounted to the inside bit of the valve of the tyre and those are much more accurate. They aren’t that expensive either. Each sensor is about €20 and lasts about 7 years on its battery before it has to be replaced. (On ours, battery is integrated, so not replaceable). If we had bought our car new, it would’ve cost us €15k-20k. Not the most expensive car. :)
