I'm putting a quick article together on how I built the quiz, but this screenshot should explain the trick behind the irregular video shapes: http://d.pr/i/1d6rc
Basically, the cover screen is a mask with 3 holes punched out to show part of each video.
Thank you Bola for asking the question, I was also wondering. I have been trying to fill a shape with a video as in PPT but kept failing in the production of this. The snapshot has been helpful David.