It's not actually. Most digital cameras pick up infrared light, especially cheaper ones. Maybe not professional cameras, which might have an IR filter for better color accuracy. The filters are not cheap and IR doesn't affect the image quality all that much.

Try it out; point your TV remote at your smartphone camera and push the buttons. You might be able to barely see the light with your eye in dark conditions, but the blinking pattern should be bright and easy to see through your phone.

Depends on whether your camera is equipped with an infrared filter. Some older camcorders actually have low-light modes based on infrared or near infrared light. You should be able to use those as well.

No, it's not iPhone specific.

You can always test yours with any IR remote control - just point it at the camera, press any button and you should see the LED blinking.

You can test it with a classic remote control where you can see the beam when you press a button if you point it directly at the camera lens.

I only have an iPhone 5, it works with the front camera. The back camera has an IR Filter.