Custom Biped Entity with Visual armour & holding an Item

Using BlockBench, I created a custom model for one of my mods. The model is like a slim model, uses 64x64 skins, and just has one extra part (which should have no effect on this)

Here are my models that I made for this. 

Anyways, I want this custom model to be able to wear armour visually. I couldn't find anything on Custom Models to hold an item even, so I'm wondering if I have to do something special with the model to allow visual armour and/or holding an item. I have experience making custom models for armour. I'm willing to do raw coding for this if people can point me in the right direction.

I wished I could dissect what makes the two official models that support visual armour & holding items and figure out how to mimic that with my own custom model, but I have no idea where those are saved nor if I can even open them. 

Also because of those two models, I know there has to be a way to make it possible to make my own custom model to also be supported with visual armour & holding items. 

Thank you for reading this 😁