Relative size of subject and background is cause by distance to them, not focal length.

Also called ‘perspective’ and the only way to change it is to move the position of the camera

It does not matter if you crop an image taken with a 50mm lens to get the same area of the motive as taken with a 300mm lens from the same ‘standpoint’ - there will be no difference between foreground and background (except for grain and noise - but that’s another story… ;-)

You have to move the camera to change that.

This is often seen in movies (those shot on real film) as opposed to on video as zoom lenses are often used without moving the camera, film based often use a dolly to move the camera. The effect of combining zoom and camera movement to keep the same crop of the foreground while having a dramatic effect of the background quickly getting larger/closer (or vice versa) is really effective - also in illustrating this concept.

In my early life (before taking the education as a photographer) I was really liking wide angles as it brought ‘life’ in to a lot of pictures. Wide as in 24 mm for my 35mm camera (Nikon F2, from 1973 should you wonder) was a favorite, replacing my 28 mm.

Too bad full frame digital is still so expensive. Using a 14-24 f/4 on the DX format in (Nikon D7100) just is’nt the same.

So now the iPhone is the most used camera (you know - the camera you have with you…!)

If you have 50mm lens, try it. It will look exactly same as if you are looking through a tube with a naked eye.

Both, actually.