Where is the aperture on a camera?

The iris, which sets the working aperture, is located inside the lens.

The aperture numbers are a ratio of the length of the lens to the diameter of the front element, or the amount of the front element the iris is allowing to be used. Let's say you had a 50mm f/2.0 lens. The aperture of the lens with the iris wide open is 1/2 the focal length, or 25mm. If you also had a 100mm f/2.0 lens the front element would be 50mm in diameter, and a 1000mm f/2.0 lens would have a 500mm front element. (It would come with a free forklift, but that's a different story.) If you set the aperture ring to f/4.0, your 50mm lens would bring in as much light as one with a 12.5mm front element, the 100mm lens would act like it was 25mm in front and the 1000mm lens would act like it had a 250mm front element, but you'd still have to lug it around.