Base skeleton without skin geometry

At the beginning of the New Animation Features in Unreal Engine | Unreal Engine 2019 talk Jurre mentions that a Fortnite character skin is composed of multiple skeletal meshes and that the base skeleton is usually called the invisible one because it doesn’t have any skin geometry. He goes on to say that the individual character parts for a character get layered on top of that and that there are several different sizes of these invisible skeletons for different sized characters.

It sounds like there is only one bone hierarchy that they all share but I’m not sure how the different proportions are setup for tall or short characters. Is that handled by the different parts of the skeletal mesh when they are imported? Are the bone transforms programmed in C++ somehow? Also does the head skeletal mesh have the entire bone hierarchy when imported or just the bones that affect it’s part of the mesh? In other words does the head have leg and foot bones associated with it?

New Animation Features in Unreal Engine | Unreal Fest Europe 2019 | Unreal Engine

The part I’m referring to starts at 0:41