I salvaged a 2010-model MacBook Pro trackpad, soldered on some USB wires, and made it external. I was actually more interested in the keyboard, but discovered that I needed both because the USB controller is on the trackpad.

https://hawkwood.com/archives/743

Here's a closeup of the chips:

https://hawkwood.com/wp-content/uploads/2009/09/IMG_3578.JPG

CY8C24794: Converts SPI to USB. [1]

SST 25WF020: 2 MBit serial flash memory. [2]

BCM5974: Multi-touch controller. [2]

TI CD3238: Driving IC. [3]

[1] https://www.insanelymac.com/forum/topic/333175-macbook-pros-...

[2] https://appleinsider.com/articles/10/07/29/teardown_of_apple...

[3] https://www.semisrael.com/blogs/technology/ti-touch-screen-c...