Infrared Cameras - How do they work?

Why do infrared flashlights emit red light that is visible to the human eye? Isn't infrared radiation invisible to the human eye? I'm asking this because I've seen this red light many times, on security cameras with infrared illumination.

I don’t believe this fully answered the question. what is an infrared camera made of? how does each element detect heat? how can the heat be recorded and digitized?

But they don't detect air; strange.

You have said that the detector sees the infrared radiation. But where does this comes from? Does the bike(or any object) emits the infrared radiation or we from the camera incident any light or etc., to reflect it back and measure with it?

How come these cameras measure the temperature of the "things" that are far away and not for example the temperate of the air in between?