how to give an entity an item and display it in their hand

Only possible with biped models

I don't think this is possible without modifying the code of the entity itself through the "edit as code" button.

Besides using Biped model, of course.

Unfortunately, this doesn't appear to be possible without custom code.

The biped models appear to have special code on their models that update based on what they are holding.

This, the only line of code I could find that represents this, doesn't even reference the model: this.setItemSlot(EquipmentSlot.MAINHAND, new ItemStack(<insert_mod_item_here>.get()));

Even looking in the renderer code, it doesn't reference the mainhand/offhand item models.

another option, and hear me out, is to make the entity model with the item in-hand already. Unfortunately, this would mean you can't use bows and the item can't be swapped or randomized.

Other than that, only Biped entities can hold items in-hand

Dear user, Is the entity you want to display the item in the hand of custom?

TheVoidShadows, could you elaborate on how could that be done?