Also how is this process done? I am not familiar with this particular practice.
We are talking about thin rubber soles (4mm; 1/8"), which get glued, on top (or underneath, depending, literally, on your viewpoint) of the existing leather soles. The original soles stay in place. It has to be done with either new or relatively little worn shoes, because, once the soles are really worn down, you would need more drastic surgery: remove old soles, fix new ones.