How Face ID works, in plain language
Face ID debuted with the iPhone X in 2017 and replaced Touch ID on most iPhones. It relies on a set of sensors collectively called the TrueDepth camera system, housed in the notch or Dynamic Island area.
When you glance at your phone, a flood illuminator sends out infrared light so the system can detect a face even in darkness. A dot projector then casts a pattern of more than 30,000 invisible infrared dots onto the face, and an infrared camera captures how that pattern is distorted. The distortion encodes the three-dimensional shape of your features.