How it Works
Blockbench .geo files are read by WSEE, and the data is parsed. .geo files describe how bones are nested, where cubes are relative to bones, and where textures are applied on cubes.
Once the Blockbench file is read, this data can be used to construct a Resource Pack. This is done by turning each bone in to its own item texture, creating a mapping of Bone Name -> Item ID, and reposition the bone within the new item so that it is centered. Centering the item is important for reasons that will be discussed below.
This system creates entities for each bone, and puts the bone texture on each entity's head. In the image above you can see this in action, using armor stands. Zombies can also be used, and provide benefits that are explained here
Whenever an animation is run or an entity is moved, each entity calculates the position it should be at and the rotation of the head. When done properly, this can make a multipart entity appear as if it is one single entity.
Invisible slimes are used as hitboxes. Attacks are forwarded to their master entity.