At what step is the difference made between a quesadilla and burrito?
Quesadilla comes from queso, cheese, and that is mostly what goes in there. Maybe some jalapeños or onions, but that's really it.

In a burrito you'd add much more, both volume and variety, for example meat, rice, avocado, cream, cheese, chiles, and beans.

As far as I know the quesadilla goes in the pan on heat, whereas a burrito does not get heated after being wrapped up.