A single, elegant chip resides on the touchpad.

Broadcom BCM5976 Touch Controller (part of the same series of Broadcom touch controller chips used in many Apple devices)

The strain gauges, mounted on each of the four spring tabs, are used to measure the force applied to the trackpad surface.

As you press down on the trackpad, the springs deflect, stretching the strain gauges and increasing their resistance. A microcontroller measures that resistance and determines the appropriate level of haptic Taptic feedback to deliver.