The answer is it varies and isn't necessarily disclosed by the individual companies to the general public. A common approach is that there is a face matching s

The answer is it varies and isn't necessarily disclosed by the individual companies to the general public. A common approach is that there is a face matching service which opaquely returns results to end users hiding much of the specifics. At this point the recognition by virtually all vendors is being done with a variety of different deep neural networks through a library similar to the popular tensorflow library. For some insights into the variation between different vendors, the recent FRVT reports may be of interest, though they can be hard to parse through: https://www.nist.gov/programs-projects/face-recognition-vendor-test-frvt https://www.nist.gov/programs-projects/face-recognition-vend...