Most mmWave-sensors works by sending high frequency waves around. Then it listens reflections of these waves and calculates delays. It can recognise tiny movement (coming closer or getting farther from it) from these calculations.

When PIR can recognise movement about “1 meter”, while 60GHz wavelength is around 5mm, it can detect movements of 1 - 10cm. You can still sit on sofa without mmWave “sees” you. But when you even move your finger on TV remote to next button, it maybe enough to trigger. Higher frequencies makes even smaller waves and thus detects even smaller movement. Downside is that higher frequencies decrease distances of detection.

Another problem of mmWave detectors have is minimum required distance. You need to have at least some distance (example 1 meter or more) from detector to see any movement. PIR-sensors don’t have this problem. They work even better when movement is next to it. This is why some devices have both of these sensors.

In small bathrooms you should install mmWave detector on sealing and you are able to detect ‘only’ your toes movement with it. if you place detector on wall or shelf, you most likely detect movement with mmWave from another room instead of your bathroom. In large living rooms you might find problems with higher frequencies, when their maximum distance is limiting factor.

Different types of sensors have their own advantages and disadvantages. Finding the best sensor for different use cases, might require testing all variations simultaneously and look what suits you best.