For one it uses a numbers of sensors: https://support.apple.com/en-us/HT208108 https://support.apple.com/en-us/HT208108 But also to generalize to makeup and occ

For one it uses a numbers of sensors: https://support.apple.com/en-us/HT208108 https://support.apple.com/en-us/HT208108 But also to generalize to makeup and occlusion and millions of faces you have to do much more. There are hundreds of papers on the subject.