Essentially, I want to know if and how this is possible. I'm trying to achieve something similar to how wheat or seeds works on cows and chickens respectively. I looked through their code but I couldn't figure out how it worked.

You want to make your own custom item that interacts with vanilla entities?  Or you want vanilla items to interact with custom entities?  Or you want to change the way that vanilla items interact with vanilla entities?

 

I ask because the approach may differ depending on what your answer is.

if you tried looking into the items look at the entities and vice versa