Can you tell me how you made this magnificent thing. VR Chat confuses me

You have to have a certain version of Unity, The VRchat SDK, A model/ modelling program and the ability to rig that model with your own tools or Mixamo (A website)

Basically you look of VRchatSDK and it gives you rules on how to set up a basic avatar. Tweaking it and getting it to run is the hard part