> How do I modify the code of that which I don’t have access to?
Native code reverse engineering is very time consuming. It’s often possible to achieve similar results by focusing on the code which you have access to. You don’t have source code of Windows OS components, but you do have their APIs and debug symbols, and that’s much better than just binaries.
If you want to change what’s rendered, you can replace the GPU API with a wrapped version, like renderdoc does. If you want to change what’s loaded from disk, patch game files, or replace whatever OS file I/O APIs is used by the game (DLL injection, then MinHook or Detours).
Even when you do need to change game’s own native code, directly patching machine code is rarely a good idea, very hard to implement and especially debug. An easier way is replacing complete functions with API-compatible replacements implemented in your DLL library in C++. Again, use MinHook or Detours to replace the implementation. C++ allows unrestricted memory access so you can read and write everywhere, here’s working examples: https://github.com/Const-me/vis_avs_dx/blob/master/avs_dx/Dx... https://github.com/Const-me/vis_avs_dx/blob/master/avs_dx/Dx... I didn’t have source code of these C++ classes, but wanted their data regardless. Found the offsets by using VS debugger, these third-party DLLs include GUI to change the values, I compared memory before/after making changes.
> What reverse engineer projects are good for beginners?
In the context of modern Windows games, assuming you wanna change what’s rendered, a good start might be https://renderdoc.org/. Officially, the tool is only supported when you run your own code. Technically, it often works with retail games too, just don’t open issues about that, they’ll be closed as a not supported use case. As a nice side effect, you’ll learn a thing of 2 about Direct3D. The tool is open source with good license (MIT), so you can fork, disable their frame captures, and change their API wrappers to modify the output of some particular game.
One more thing, modern games use a lot of bytecodes. E.g. D3D shaders are byte code, search “3dmigoto decompiler” to decompile dxbc into HLSL. .NET is often byte code (Unity3D is based on .NET), use reflector to decompile into C#. Many games use custom VMs, sometimes modding community has decompilers for their custom byte code.
> I’d love to pick up an older gadget and try to reverse engineer it
What do you mean by “gadget”?