I was using unreal back when it was version 2.0. Sockets existed in the engine back then. And the base skeleton didn't look like this. It had no twist bones - for example.
Every skeleton has a root bone. The root bone is simply the first bone in the hierarchy. It can be a bone that is not weighted to the mesh. In this case you can use it as the origin, to manipulate the rotation of the rest of the skeleton by keyframes at the skeleton for example. That a extra root bone gets added…