Each camera has a different optical system with a different focal length range. This lets the camera cover everything from ultra wide angle to telescopic magnification while remaining thin enough to fit in a phone.

Phones can't fit a zoom lens with moving elements. The other cameras are just other prime lenses so you aren't restricted to having a single focal length all the time.

When you hit the little 0.5x, 1x, 4x buttons in the camera app you are switching between the physical cameras. Though Apple goes out of their way to obscure what is actually happening.

The obvious issue with the product here is that the telephoto lens will not cover the whole image that the 1x captured so only some portion of the center of the frame will be higher resolution.