To maximise screen space, many phones moved the fingerprint sensor beneath the display. When you press the indicated spot, a sensor under the glass reads your fingerprint, removing the need for a separate button or a rear sensor.
This keeps the front all screen while retaining quick, familiar unlocking, and it can be combined with face recognition for flexibility. Under-display sensors vary in speed and in how well they cope with wet or dirty fingers, and the technology differs between the optical and ultrasonic approaches used, but it has become a common way to fit secure unlocking into a clean design.