So, I just made a game, but I decided to make a game launcher in visual studio or unity to launch the game, but is it possible to set the game variable inside the application?
Yes. Ermmmm… Maybe… You need to have some sort of test file that unity can read and the launcher can write to. I would recommend (like the minecraft launcher) make something like that, that has adjustable screen size and resoultion etc settings. Then have it log to that text file and in code tell unity to read that and set the Screen.width and height to use those and tell unity to read the other settings too so you can have like how many What ever you want