There are a lot of steps that need to be done actually.
- Download and pray that it is correctly rigged. (Rigging is the term for giving a mesh a skeleton, same as our bones.)
- If it is not rigged, which is most likely the case… you are in for a looong session of giving life to your asset. Look up some Rigging Tutorials and keep in mind, that this is a long process if done correctly. Depending on how serious you are, or how often you are going to rig stuff, you might want to check out AutoRigPro.
2.1) just a quick tipp, try importing your mesh into Mixamo. They do an awesome job with automatic rigging on humanoid characters as well. No promises tho.
This should also already fix “weightpainting”. The process of deciding which bone controls which geometry. If not (and since your character has a lot of clothes this is most likely the case: you gotta adjust it yourself.
After all these steps, you can export, or rather import your mesh to UE5 and do all the fun UE5 shenanigans.
Making a character animation ready is hard work (if done properly). There are many ways to cheese it and make it easier, but there are quality tradeoffs.