Buy The End Is Nigh

You need to use your your game CD Key on the Playstation Network to download your game. To access the Playstation Network, you need a Playstation 4, a broadband connexion, a PSN account, and a storage device according to your console: Hard disk drive, memory card, or USB drive.
Playstation Plus Subscriptions are compatible with PS4, PS3 and PS VITA.

This is Nintendo's download platform. After buying a Nintendo Switch game, you will receive a download code. Start the Nintendo eShop from the Home menu, select Settings/Other, and then tap "Redeem Download Code" to enter your 16-diogits code which will allow you to download your game.

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.