Custom mob holding an item?

Hello. I have recently been wanting to make a variant of the Illager called a Poacher, with its own structure in the forest yada yada. However, the original plan was for the Poacher to carry a crossbow in the main hand and firework rockets in the off hand.

I have been looking for a while, trying to find a solution that actually is a solution, not just adding it to the model, and a solution that isn't from 7 years ago. I am aware it will most likely take custom code, but I just need to know what that code is. I have looked on a lot of forums where people just say "custom code" and don't specify.

Please help. I'll give all the info you need about the mob.