It’s on www.GamestoGrunts.org, and you have to be able to verify on the ID.me site to register for GamestoGrunts, so that means US and Canadian militaries and vets only.
Game selection varies, as they sell out of some and add new ones from time to time.
While it’s currently a limited selection of only about 8 games on Steam (PC), you can see on their list of sold out games, they have had quite a few different popular games on various different platforms, some of them being regular $30, $40, $50 games.
You get a digital code to download the game for free from whatever platform the code is for. Sometimes PC, Xbox, PS, or mobile.
As for Stardew Valley, it may be “just” a farm type game, and it may be a 5 year old game, but it’s still in the top 20 most played games on Steam. That’s how popular it is.