it looks like that rig is a series of empties that have been parented together. the reasoning for that is anyone’s guess, but since it came from the unity asset store and imported into blender it could be any number of things - there could have been an export error, it could have had some additional components that don’t transfer out of unity, etc. off the top of my head that figure looks like a slightly disproportioned version of the 3ds max biped.
re-attaching the body is just a matter of aligning the two and then going through parenting each part of the body to its corresponding empty. tedious, but given the simplistic character design still pretty straightforwards.
the bigger question is what you are hoping to do with it. if you’re looking to practice animating, downloading a skeletal asset intended for another software usually isn’t the most effective way. look on blendswap for some free character rigs that’ll have animator friendly setups in blender. or if you’re committed to this mesh, you’d probably want to watch some tutorials on rigging or using blender’s rigify tools to generate something yourself.