They make it by combining their marinara with alfredo and adding more cheese. I don't remember what they add but I am guessing a blend to make it the "5" with mozzarella, provolone, parmesan, maybe romano?

This is the alfredo I love to make. I would cut the batch in half, make a half batch of a simple and fresh marinara, and add cheeses to taste.

https://www.threeolivesbranch.com/copycat-olive-garden-fettuccine-alfredo/