Setup
The setup process depends on the where the game was bought.
PC / Mac
Steam Store
If the game was bought via the Steam store, it is automatically added to your Steam library.
Setup and start the Steam Client, double click the game in the library and follow the instructions.
GOG
If the game was purchased via GOG, it is automatically added to your GOG library.
You may either use the GOG Galaxy Client to manage your GOG games or manually download the standalone installer. All possible ways are described in the GOG Support Center.
Epic Games Store
If the game was purchased via Epic Games, it is automatically added to your Epic Games library.
It can be installed via the Epic Games launcher.
Product key activation
If the Steam product key was obtained from a 3rd party website, it can be activated in the Steam client: Activating a Product on Steam.
Console
Xbox
If the game was purchased through the Microsoft Store, it is automatically added to the game library under My games & apps on your console.
PlayStation
If the game was purchased through the PlayStation Store, it is automatically added to your game library.