How to prevent screen size from showing more space?

The problem in Unity if there is a screen with bigger resolution than your device it changes and shows more. for example I worked on a game that has a width of 800 (computer) on mobile since it has more width it reveals more things that it shouldn't show. I want the game to scale not show more. I would like a letterbox scaling for the whole game like in Construct 2 Game Engine (for reference)