To reply to your questions:
To update my previous post, the skeleton of a Metahuman does have 5 spine bones, plus all the corrective joints, but not the facial rig, which is from another skeleton.
The Mesh is skinned character that is using the skeletal hierarchy from the skeleton.
If you open up all the different body parts, you’ll see that each one has the same base skeleton, so the MetaHuman is built by different body parts, that you can also change if you want, which is quite comfortable, since if you have a character where all the body parts ( face, boots, pants, and so on ) are a single mesh, it’ll be quite annoying to update.
Open a Metahuman base BP ( the one that comes with the downloaded MetaHuman ) and look at the “body” skeletal mesh.
This is the “body” skeletal mesh ( note the top right selection ), and as you can see there’s just the hands and the calfs
This is the Skeleton ( note the top right selection ), where I do have a preview mesh called “f_med_nrw_body”
If you’re not seeing 5 spine joints, I’m not sure if you’re looking at the Mannequin or something else.
Regarding the facial rig, the face has its own skeletal heirarchy, which does share joints with the body rig, so that using the face as a child of the body, and using the Copy Pose from Mesh node inside the AnimBP, allows the face neck/head to inherit the animation from the body, while the facial rig does its own thing.
The combining of the head+body is already done inside the AnimBP of the face, so if you decide to record mocap directly inside Unreal, you can use Take Recorder, and it’ll take care of recording your performance as a simple animation clip.
If you also want to record facial animation ( with an iPhone or Faceware/Facegood ), you can also do that together with the body, sicne the animation will be saved separately from the body animation, since it’ll save just the face joints animation.
You can then use Control Rig to bake the animation that you previously recorded onto the rig itself, it is a very simple process, so you could have two separate animations, one for the body and one for the face, all baked onto the controls of the Control Rig.
Honestly it would be better to tweak/cleanup the mocap recording using an external DCC ( Maya, MoBu, whatever ), since as of now the tools available within Control Rig are not ideal for cleanup, same thing for the facial animation.
I do use Unreal as my “all in one” mocap recording tool, but I then export all the animations in MoBu, clean them up, and the reimport them inside Unreal, then use Sequencer to add the anim clips, audio and camera movement.