There is no need for a special syntax for creating windows. All that is required is that the OS provides an API to create windows. Such an API consists of simple function calls for which C++ does provide syntax.
Furthermore C and C++ are so called systems programming languages and are able to access arbitrary pointers (which might be mapped to some device by the hardware). Additionally, it is also fairly simple to call functions defined in assembly, which allows the full range of operations the processor provides. Therefore it is possible to write an OS itself using C or C++ and a small amount of assembly.
It should also be mentioned that Qt is a bad example, as it uses a so-called meta compiler to extend C++' syntax. This is however not related to it's ability to call into the APIs provided by the OS to actually draw or create windows.
