Low human skeleton. PBR / BSDF textured using the lilysurface scrapper Blender addon. Built using Blender. Textures are from CC0 textures with some modifications by me.
File includes two versions of the same model. One is a rough, more pitted version of the texture meant to look more decayed. The other is a smoother, cleaner version.
I tested uploading to Mixamo and that worked well.
The polycount looks very high in the Sketchfab preview, but if you download the .blend file, the model parts are all attached to subvision modifiers (they looks much better in preview that way).