Loading models is done via Resourse Packs.
Models are loaded with item models under "namespace/items/item_model.json"
If you made a item model and saved it as "cct/items/backpack.json" (this is in the mod by default),
then the model should be loaded like this in-game:
!Example for setting up a item model in game (Animations are supported)
This can be done via the config, or Resourse Packs, but this time you want to make a new folder called "back_models" under your namespace, then put in a new json named after your item.
(Example: "minecraft/back_models/diamond_sword.json")
Inside you will be able to set a item model to render when that item is set as a back-item.
Example:
!Example of code to put in the new json file