I don't mean this flippantly, but it's usually whoever you're writing the check to. My first home had the mortgage sold within a week, and someone else started taking the money out of my account.
Some originators will hold on to servicing of your debt wherever it goes (within certain boundaries, e.x. if you go into a delinquency or reconciled delinquency state).
Those originators use it as a selling point, because many people have gotten bit by servicer flips one way or another.