Guide on adding a new model, animation, and creature to HoMMV

Let's start with the current state of AI – what prevented the developers from simply taking the AI from the third game instead of reinventing the wheel in the next four installments (this applies not only to the fifth game).

I don't want to participate in arguments, but... How is it possible to "take AI"? It sounds paradoxical; it's unlikely that the AI in the third game is a universal library that can be plugged in and adapted anywhere, like some DeepMind.
I fully support the creation of any guides and other content. I don't know how much longer the fifth game will last, but the fact that its lifespan is extended against all odds is a fact. The fifth installment has been alive, albeit with varying degrees of success, for 14 years, which is twice as long as the time that passed from the creation of the third game to the creation of the fifth game; however, that's a fact...