Why isn't there a designer for native api forms in Visual Studio? Similar to Delphi? If there exist some programs, tools etc, please advice.
What is the best approach to design complex windows in pure API?
That's probably because there is no standard way of doing control layouts in WinAPI, you have to manage it by yourself. There is no base "Control" class in WinAPI - everything is a Window of some sort, so no way to support their differences with a common layout editor/designer.
You can however create your window layout in a dialog and make it resizable by yourself or using methods published on codeproject (this or this - both are MFC-related, but that's fairly easy to translate).
Or adapt ScreenLib to your desktop needs.