I have been interested on win32 programming via pywin32 in python, but the document seems to be vague on how to create splitters, buttons and other things inside the main window.

As far as I have understood, any default UI buttons and other should be done inside WM_CREATE that is implemented before window becomes visible.

There is win32ui.Createsplitter, but how can I understand the use of it compared to C/C++ code?

class Window:
def window_proc(self, hwnd, msg, wparam, lparam):
    match msg:
        case win32con.WM_CREATE:
            pass
        case win32con.WM_CLOSE:
            win32gui.DestroyWindow(hwnd)
        case win32con.WM_DESTROY:
            win32gui.PostQuitMessage(0)
    return win32gui.DefWindowProc(hwnd, msg, wparam, lparam)