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Seeing as I only use Steam these days:
Go to the library page for the game question and scroll down past DLC - you should see Steam Workshop. Click Browse the workshop.
Welcome a whole new world of stuff from updates to graphics skins, AI tweaks, weapon packages, all sorts depending on the game. Not sure what the 2009 mod is on Stalker but some games have TONS of modded content - usually free though recently Steam have allowed creators to start charging for it.
I don't understand mods. I'm new to this PC gaming lark. Please elaborate.