Character's & FBX?

I do understand the process of importing Characters using an FBX files that contain mocap data.
Characters do have a rig & the animation can be loaded into Unreal.

What I don’t understand:
Free UNREAL Characters, that you can download for example at the FREE EPIC Marketplace.
Those Characters do have a Rig - and are fully setup for Animation.

How can I load animation data (mocap files in .bvh or pre-recorded FBX files) to one of those characters?

Mixamo has pre-made characters with animation data for download.
If I download one of those characters it contains the fbx data - and the motion capture file.

Can I swap out the mesh and use the same animation for a character that is for FREE available at the EPIC Games Marketplace?
And if so, is there a guide or tutorial that shows that process?

Thanks a lot for more information & clarification!

First you need to retagret the skeleton of your custom character to the UE4 Mannequin, then you can retarget the animations you need.

Here you’ll find how it works

yes… thank you. Is there anywhere a documentation that shows what bone has to be re-target to what specific bone?
How to know to make the right selection?

Ehm…just look at the name of the bones, it’s pretty obvious

that’s right… but even when doing so… it’s not working. After going step by step through every single bone… it’s broken.