Can someone please explain collegiate football spring games?

Everybody seems to make a big deal of the first "game" of collegiate Div. 1-A football programs. And I realize that these teams are playing against themselves. But how do they "work"? I realize that it is probably the teams' starting offense against the teams' starting defense. But when the offense is playing defense, it's obviously, I think, not the offensive line playing defense. Just like when the defensive line gets the ball, it's probably not the defense playing offense. So who plays the opposite side of the ball? Thanks for the clarification.