If you are referring to the A.I that you hear on TV all the time like every company is saying they do, then its usually programmers doing most of the work. They may have mathematician or some other specialist working with them. Basically, their job it to take mountains of data , feed it than A.I programming languages or library to arrive at correct assertions about the data on the other end. The process of turning the knobs to help the A.I language come to the right conclusions is where the hard work is. It's ultimately the "model" that is used that gives A.I its power for specific use-case