>>364044939
find avatar files under \AppData\LocalLow\VRChat\VRChat\Cache-WindowsPlayer
Match the time you downloaded someones avatar files with folder creation dates. Look for _data files in sub folders, those are avatars and maps.
Find some Unity ripper program like uTinyRipper or AssetRipper (google for their githubs)
open the _data files in ripper and extract everything, then import what you extracted into unity (usually named Assets folder)
You should have everything from animations, controllers, materials, textures etc. including the prefab for avatar. Sometimes things are broken on prefab which I don't know how to get around but no one else is responding. I think I heard using newer unity versions first and importing back into vrchat supported unity works(?) but never tried myself.