Steam shows both the minimum system requirements and the recommended ones. How are the latter determined?
Most games have settings ranging from a bare minimum to some form of ultra mega. What settings are typically used when determining the recommended system requirements?
From the Sub-Point “Releasing” of the Steamworks Partner Page:
Then scroll down and enter the corresponding system requirements. Preview your store page and then publish your changes when you’re happy with how the system requirements appear.
So the answer is: they come from the game developers.