How does a 'camera detector' (/hidden device detector) detect the hidden camera/device? (curious as to whether they give many false-positives when used in real settings, e.g. airbnb or hotel room).

It's an RF field strength meter. I watched about a half minute of the video then shut it off.

what other comments said + characteristic reflection of camera lenses observed through the snake-oil-looking dichroic sighting glass.

RF or EMF detection, magnetic field detection and an IR light plus filtered lens for visually checking for camera lenses.

Ye old k18 is still the $50 go to for foiling airbnb perverts.