Simplest way to verify, launch visual studio 2015, press Tools/Spy++, in Spy++ find the visual studio’s main window, you’ll see that Win32 thinks visual studio is a single window with no children, with class starting with “HwndWrapper[DefaultDomain;”

Unlike that, Spy++ itself uses Win32 (wrapped by MFC but it doesn’t matter) for GUI, you’ll see these SysTreeView32, msctls_statusbar32 and other native controls there.

While interop is indeed available if you need to consume a Win32 control (not without limitations but generally works), by default WPF doesn’t use native controls. Instead it paints stuff on GPU with Direct3D 9, and on top of that implements its own markup, styling, layout, animation, behaviors, composition, etc…