I would also be really interested in this. One thing I did figure out by poking around and Googling a bunch is the purpose and mechanism of the corrective bones:
If you open the post-process animation blueprint (under the “Rigs” subfolder of the mannequins) you’ll see that the anim graph is just a chain of Pose Driver nodes. Select one of them, and in the Details panel under Settings (at the very bottom) you’ll see a Pose Asset assigned. Open that, and underneath the asset viewport you’ll see a table of pose names and weights. Set one the weights to 1.0, and you’ll see the mesh assume that pose in the viewport.
This setup is called an “RBF” (radial basis function) system, and the way it works is a lot like corrective blendshapes/shape keys/morph targets, except instead of correcting the deformation by directly moving the vertices, you correct the deformation by adjusting those corrective bones. (This is nice because you can (theoretically) reuse the same corrective setup on different meshes.) Basically, you have one driver bone (e.g., thigh_x) which drives the position of several corrective bones (e.g., the bones under thigh_x > thigh_correctiveRoot_x) by interpolating and blending between their positions within those different poses in the pose asset.
That said… I tried to set up a skeleton following this template in Blender, but I had a hell of a time trying to bind the mesh to the rig with the right weights to get the corrective bones to work as intended. This is probably because I have no idea what I’m doing (I’m an engineer), so your mileage may vary.
I hope they do a Feature Highlight or Inside Unreal stream diving into the new mannequins and animation features soon! It all looks very exciting but also pretty overwhelming.