Every game is its own stack of hacks that barely works, usually; so knowledge about one often doesn't help much with another. The universals are things like 3d

Every game is its own stack of hacks that barely works, usually; so knowledge about one often doesn't help much with another. The universals are things like 3d modelling (but with tweaked not quite standard file formats too often), Lua scripts (lots of game engines (mis)use Lua a lot), and a general knowledge of how 100,000+ files might come together in one place to make your computer do something. An experienced programmer should be able to find their way pretty well. Find a game you like (preferably that has a modding community already), decide to make it do something it doesn't do yet; and go to it. Find an existing mod that's close to what you want and try to make it stretch to your vision. Look at all the mods for your game you can stand to, and see how others write theirs. Be prepared for some horrors due to non-experienced programmers "getting it done" with no idea how or what they're doing. Then go find a different game and discover how varied the experience can be :) Factorio is great for diddling with, very straightforward, fully (and excellently!) documented, voluminous examples of all manner of ideas already implemented.