This is a little more informative, but you still can't tell what's really going on, because it's magic. If you could peer inside the My.Forms feature, you might find Visual Basic adding parallel form ...

Although C# markup for Xamarin.Forms has been available since Xamarin.Forms 4.6 (it's now at v4.7) debuted this spring, it was just "introduced" on the Microsoft Developer Blogs site by creator ...