How Do You Make the Game Fit On the Screen?

Hi, I’m on section II, building the game “Breaker”. I’m loving the book so far, but I don’t understand how the game elements are made to fit on the screen so well. Can someone please
help explain how the Position and Length for the barriers, and the
Position and Euler for the cameras, are determined for a game like this?
Is it trial and error experience, or are these values calculated
somehow? It seems that the given values provide optimal visibility,
across devices, but I don’t understand how.