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He used Renderdoc, which lets you pick a frame apart draw call by draw call, inspect shader bytecode, etc.

He calls out Renderdoc for picking out rendering internals in the third part of the series: http://www.adriancourreges.com/blog/2015/11/02/gta-v-graphic...

You can poke around D3D11 and OGL games, though I haven't tried out the OGL functionality. Next-gen api support is coming as well.

If you've poked around other engines, either via debuggers or actual engine development, you actually run into a lot of the same concepts as far as piecing a frame together. The interesting parts come in the details of those sub-passes, which is what gets analyzed by the article author.

I suspect he's using graphics debugging tools like RenderDoc: https://github.com/baldurk/renderdoc