https://www.theregister.co.uk/2019/07/30/google_project_soli_coming_to_pixel_4/

Google will include motion-sensing radar in the forthcoming Pixel 4 smartphone to enable gesture control by waving your hand.

Project Soli uses electromagnetic waves to detect the size, shape, material, distance and velocity of nearly objects. The system is optimised for motion resolution to detect things like finger movements. Examples on the Soli website include sliding your thumb along a finger to simulate a slider control, or twirling a virtual dial by rubbing finger and thumb together.