The NCAA only allows a certain amount of on field coaches (10, it used to be 9) & can have a certain amount of full time assistants & GA's, but for QC's depending on how they're designated you can hire as many as you want.
The main job of a Quality control coach is to watch, breakdown & analyze film for the upcoming games, they chart tendencies & plays for opposing teams for their side of the ball. So for example, if you're an Offensive QC you're watching Defenses & going snap by snap charting every single play to get a read on all their calls, formations, personnel & tendencies, like which downs they like to Blitz, who they send on the Blitzes, how they like to create pressure, which coverages they use vs certain Offensive formations, how often they audible based on what the offense does at the LOS, what they do in Cover 2, Cover 3, Cover 5, Cover Zero etc..
The same for Defense, the QC watches every single snap of opposing offense they're going to be playing & they're chartinf analytical tendencies based on formations, audibles, play calls, pass protections, how the QB checks in & out of plays, how often they go in pre snap motion, when they like to run the ball, which plays they use in the red zone, all their situational no huddle calls in 2min offense & late game drives etc..
QC's have the responsibility of watching game film for all the teams they'll be playing, weeks & sometimes months in advance. They also analyze practice film for the Coaching staff & give updated analytical data & charting for which players are progresses/regresses throughout the year.
But the difference between QC's in college & the NFL is in college QC's can have multiple responsibilities more than just watching film for the season, but also they help in recruiting as well. They're not allowed to off campus recruit, but they can contact players & deal with them on campus.
Analysts, QC's & GA's all basically help with the day to day grunt work in assistance to the Position coaches, they help with anything & everything that needs to be helped with & it's great experience to become a full time on-field Coach.