Traditional SLR cameras do phase detection autofocus by directing light onto AF sensors in the camera�s mirror box. Some newer sensor technologies do away with the need for separate sensors by building the phase detection technology directly into the image sensor.
If you�re curious about the technical details of how these things work, Toshiba just released this (relatively) easy-to-understand 3-minute video that explains how phase detection autofocus works, both with separate AF sensors and with phase-detection sensor pixels.
Started out doing photography at the age of 6 using an uncle's old 1940 kodak brownie box camera. At 15 years of age, I decided to buy my very own 1975 Praktica SLR camera. I now shoot with a Nikon D850. I do unpaid TFP and commercial paid work.