Im on windows, so I don't know how it'll be differen't for Mac or Linus (I also haven't downloaded this specific game yet lmao), but based on how i've downloaded other games there should be a new folder (It'll look different from most folders as it'll have a zipper icon in top of it) in your downloads section titled Among Us or something similar I'd assume. double click on it to open it, then inside the folder look for a file that's titled "Game" and the file type says "application" double click it and it'll say that you need to extract the file. Click extract all and wait for a bit, for me it generally doesn't take long. Once it's extracted a new file will appear, this time without the zipper icon, double click that to open it, then look for the file from before titled "Game" with file type application, then you can double click it to open the game.