Take a look at the IK Rigs that come bundled with the UE5 Templates and Metahumans.
Mannequin Rigs are under Characters → Mannequins → Rigs → IK_Mannequin
Metahuman Rig is under Metahumans → Common → Common → IK_Metahuman
There should be retargeters in the respective folders as well. If you want to use any other animation packs that use different skeletons, you’ll have to create an IK Rig for it and create the chains and goals yourself, then create an IK Retargeter for the actual animation export. For the most part, it’s just copying the settings on to the new Rig and Retargeter by hand.
This is probably a good starting point:
https://www.youtube.com/watch?v=9cKuEXFDwl0&t=161s&ab_channel=Jobutsu
That’s the most I can give you at the moment, gotta go to work.