Buy GNOG

For Steam CD Keys, you must download (HERE), install the Steam software and create a new account. If you already have a Steam account, once you log in, click "Add a game" (located in the bottom left corner), and then "Activate a Product on Steam". After a few more clicks, you will be prompted to type in your game code. Thus, the game will be activated and automatically added to your account library.

This is the Epic Games launcher. You must download the client HERE, install it, and then create your account. When you sign in to your account, click on your avatar and then on "Redeem Code". Enter your Key, your game will automatically be added to your library, and the download will start.