C/C++ What should I focus on learning?

[Phenomenal]

 

[panda]

The best way to learn how the more complex applications work is to study their code and tinker with them to see how behavior changes.

 

I would highly recommend tinkering with Mana Source/Mana World if you want to learn more about this stuff.

I highly discourage looking at any NeHe tutorials (or anything similar) unless there are updated ones for GL3/4 or GLES3.

I also highly discourage learning Direct X, particularly if you're going to learn below DX12.