Departures from the USA are part of a joint program with US CBP.
Photos for US Citizens are provided during their passport application. Photos for foreign nationals are those collected by CBP on arrival in the US.
The flight manifest / passport information is passed to US CBP prior to boarding commencing. CBP then cache all the known photos of those passengers. As you approach the boarding gate, the system matches your photo against that cache to see if you are expected on that flight. If found, the details are passed back to the airline's Departure Control System to board the passenger, and access is granted.

Due to the need to have existing photos, this system is currently only available ex-USA. However, back in October 2018 they were looking at trialing it on the A318 service from LCY due to the likelihood that users of this services were previous visitors to the USA.

Departures from the UK use a different system (to the best of my knowledge).