The dots are invisible to your eyes. But you can see them through an infrared camera. The iPhone X's camera analyzes the dots on your face and compares it to the image made when Face ID was set up.

Apple's rumored foldable iPhone currently in development features an under-screen camera integrated into the folding display, ...

Seeing the unseen is one of the great things about using an infrared (IR) camera, and even the cheap-ish ones that plug into a smartphone can dramatically improve your hardware debugging game.