What is the principle of focusing in photography? Is it to change the distance?

I happened to see this question, and no professionals answered it. Let me explain.

Focal distance = object distance+image distance = the distance from the object to the film or imaging sensor in the camera.

The focal length of the lens group of the lens is f, and the lens group must have a focus. Camera imaging requires that the object distance is greater than or equal to 2f, and the image distance is between f and 2f. Object distance is the distance from the focus center to the photographed object, and image distance is the distance from the focus center to the film. The focusing process is to change the relative distance from the film to the focus center, that is, to adjust the front and rear positions of the whole lens group, so that the film is at the image distance of the subject. Then, when focusing from the nearest focal length of a lens to infinity, the moving distance of the lens group is theoretically f, that is, the focal length of the lens. If a lens with a focal length of 50mm can achieve the closest focusing distance of 100 mm (in fact, it is impossible =2f), then when focusing at infinity and 100mm, the position of the lens group will be displaced by 50 mm. Obviously, the focal length and viewing angle are unchanged, so when the lens group is displaced,

-An engineer who works in research and development at Leica and Carl Chase. Although I am engaged in circuits, I am familiar with the basic principles of my old club's products.