Like with toolkits such as qT, wxwidgets and such, how does an API designer provide and api that is the same, even though it calls totally different system calls to do so? For example, in Windows OS you have to mess around with a whole lot of functions in the GDI. On Linux you have to mess around with a whole lot of functions in XLib and whatever other layers the distribution has on top of in. So how how can you design an widgit kit that can unify all that functionality? so that say CreateWindow() will create a windon on any platform? I don’t comprehend how this can be done.
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Instead of using Xlib or GDI, you could use something that is more universal. For example, you could use OpenGL, which is supported everywhere. I think that is what Blender’s UI does.
Some toolkits can be modified to use some kind of backend for each platform they support. This is basically what Qt does. On Mac OS X, Qt apps use Cocoa as a backend. Qt for OS X was made specifically for that OS. However, there are other Qt implementations on other platforms, so that’s what makes Qt work on more than one platform. SWT for Java works the same way (using the OS’s native toolkit as a backend).
Other toolkits can use some kind of high-level layer to render. For example, Swing for Java is rendered purely using Java APIs, and is not tied to any platform at all.