The cash-on-delivery (COD) system for Steam games works by allowing customers to pay for their games in cash upon delivery. When a customer selects the COD option during checkout, the game is shipped to their address, and they are required to pay for the game in cash when it is delivered to them.
You go to the game page in the store, there you find the play button. Then the game starts, in the game you click on the multiplayer tab, file management and click install. After all the actions, close the game and the installation of the necessary files will begin.