you go, a tutorial about retargeting Epic’s skeleton to Mixamo skeleton:
https://.com/watch?v=u87GKmTnDu0
I hope it will make things much clearer. This tutorial will work for any animations and skeletons.
Btw. I had some thoughts about buying assets on the internet (I also buy a lot of assets). The truth is, that internet asset stores (whether it’s Marketplace, Unity Asset Store, Turbosquid, Mixamo, 3d Export or whatever) never formally signed a contract with you to store your purchases forever, in unchanged form, for the rest of your life. It is only wise, if you download your assets after you buy them and store them on your harddrive, pendrive or something like that. In the next years, Epic could go out of bussiness, Epic could close the Marketplace, Epic servers could get attacked and wiped out, the asset creator (for example me) could quit the job, or simply die. Or, as we see , the asset could get irreversibly updated to the version you don’t like.
You own the assets forever, but you should store them on your harddrive, if you don’t have any guarantees that they are stored safely. It is especially true with animations, since they don’t age as fast as 3d models. You could still use them in 10-15 years, but there is a big , that they won’t be on Epic’s servers in that time. If there is still an Epic in 15 years.
Back up your assets :).