Certain vehicles utilize indirect TPMS systems instead of direct ones. Examples of manufacturers that use indirect prominently are Honda and Volkswagen.
Since the systems measure tire pressure indirectly, there’s no sensors inside the wheels. Instead the vehicles use the data from the ABS sensors inside each wheel hub. By comparing the spin-speed data from each sensor, the tire circumference can be calculated and thus the approximate tire pressure also. Properly-inflated tires have a bigger circumference than deflated tires.
TPMS sensors aren’t required for vehicles with indirect systems but a reset procedure should be performed whenever new tires or wheels are installed, so the system can be re-calibrated.