i downloaded how do i open it and where is it
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.
If your purchase comes with a Steam or activation key then you'll find a Get key button below the list of files. When you're ready to get the key just click the button.
Once you have installed the Steam client, launch the client and enter your login credentials. When you are logged into your account, click the button in the bottom left corner that says "+ ADD A GAME…" to register the product key to your account.
In the pop-up window, select the "Activate a Product on Steam" option. This will open up a new window.
Follow the prompts, and paste or enter the numerical key you copied from your download page into the "Product Code" field.
Hit "NEXT>" If your key is valid, Steam will accept it and you will be taken to your library where you will be able to download and play your newly registered game.