Those joints are controlled via a Post Process AnimBP where the Pose Driver node takes care, using a ROM animation, to move/rotate those additional joints, so either using a live mocap performance or in game, those joints are automatically driven, giving you a more accurate deformation compared to standard skin weights from a default skeleton.
If you open up a Metahuman skeletal mesh, you’ll notice that at the bottom of the viewport it says that there is a post process AnimBP, so open it up and you’ll see various Pose Driver nodes and their setup, which is a good reference for you to check if you want a similar behaviour.
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