> He's talking about the motion detector things that are a drop in replacement for switches.

No I'm not, but those work too.

This is what I bought: https://www.ebay.com/itm/153285261299

It is described as a radar sensor because it uses a high frequency oscillator and monitors the change in the signal due to things moving in the electromagnetic field. Imagine the effect of waving your hand near the aerial of a radio receiver when the signal is weak.