They're using a pretty nifty differential Hall magnetic encoder, like this one: https://www.novosns.com/en/hall-angle-sensor-4010 https://www.novosns.com/en/ha

They're using a pretty nifty differential Hall magnetic encoder, like this one: https://www.novosns.com/en/hall-angle-sensor-4010 https://www.novosns.com/en/hall-angle-sensor-4010 There's a fun Hackaday video using the same type of part in a rotary knob + LCD control. https://www.youtube.com/watch?v=zIrAe23f8sg https://www.youtube.com/watch?v=zIrAe23f8sg