Not so much as composite, I think. Everyone knows how a regular photograph works; the camera shutter opens for a split second and captures that moment just as we would see it with our eyes. The finish cameras are different. As I understand it, there’s a thin slit at the finish line with a strip of film moving behind it. Instead of freezing the action as a rider crosses the line, it’s a scrolling scene of the riders crossing the line that’s shown as if it were a photograph.
Cut a slit in a piece of paper and move it from right to left across the picture; that’s what you’d see if you were looking at the finish line.