On which platform should I start developing?

I am very thorough with C++ and OOP. I am familiar with all the topics discussed in Bjarne Stroustrup's C++ guide. I still wonder how people write super applications like Photoshop, Windows, Maya and so on using C++. Next thing I am interested in learning is how do I create my own API's/Frameworks and SDK's like the Windows MFC, Mac Cocoa, Java AWT and so on. I am also unsure on which platform I should start developing. There are lots like Windows MFC, Cocoa, Java, Qt and so on. Basically I am more interested in how I can understand and write an Operating system in C++ and how I can create my own API's like MFC, Cocoa, OpenGL, DirectX and so on. I also wonder how people write firmwares and codes for drivers.