Retargeting animations

The generated skeleton matches UE5 Manny bone for bone, with the same names and the same orientations. That is the whole point of fitting Manny instead of building a new skeleton, and it is what makes retargeting straightforward.

Manny animations

Because the bone names and the roll match, an IK Rig and IK Retargeter built for Manny works on your character with no remapping. Set the source to Manny, set the target to SKEL_YourCharacter, and the chains map automatically.

Mixamo animations

Mixamo animations retarget to Manny, and anything that retargets to Manny retargets to your character. Bring the animation in against a Manny skeleton first, then retarget from Manny to your rig.

Proportions

Retargeting handles different proportions, but it is not magic. A character with very long arms or a very short torso will need the retargeter's settings adjusted the same way any retarget does. The rig itself is not the problem here.

What is not driven

Twist bones are placed at their parent and have no animation driver in a generated rig. If your animations rely on twist bone correction, drive them with an Anim Blueprint or a control rig on top.