ARKit face tracking is shrouded in a ton of myth and mystique.
I reverse engineered the internal face tracking API, and this is how it actually works.
This is based on reverse engineering the API of Apple's face detection framework and testing it with my own code. Some of it is educated guesswork based on names and strings. I did not reverse engineer the details of how ARKit/FaceKit works, but given what I've seen, I'm quite confident in the overall big picture.