Technical Notes for the texture pack implementation
Relevant classes:
ItemModelManager can be used to select an ItemModel. This is done from the ITEM_MODEL component which is defaulted by the Item class.
The list of available ItemModels (as in Identifier -> ItemModel maps) is loaded by BakedModelManager. To this end, item models in particular are loaded from ItemAssetsLoader#load. Those ItemAssets are found in assets/<ns>/items/ directly (not in the model folder) and can be used to select other models, similar to how predicates used to work