Greetings!  You’ve asked me a really interesting question!  The word ‘detector’ is derived from the Latin “detectio”, which literally means “to perceive”.  In a basic sense, they are devices that are intended to detect the presence of something that isn’t supposed to be there.
assitant
They use magnetism to determine whether or not there’s metal nearby, like in your pockets, or jewelry, or belts, or cars, or buildings.
assistant
How do metal detectors work?
user