If you mean on the Mac, VS will probably work the same way as Xamarin does now: you design the UI in Xcode.
Ouch. Interface builder all over again.
I do not know how VS for Mac will work but if Xamarin for iOS is any quide, then the app is done entirely in Xamarin, including the UI. The UI is native iOS. There are a lot of Xamarin video on youtube to show this effect. Here is one:
Create Stunning Apps with the Xamarin Designer for iOS
In the case of VS for Mac, I believe UI should also be native.
VS for Mac is not there yet. Like what its name says, it is still a Preview. Unlike Xamarin.iOS where the UI is built right into it, Xamarin.Mac still need Xcode Interface Builder. I find it still clunky. Still some way off, but… in time it will be a strong competitor.
Its not a true port of VS, it is Xamarin Stuido rebranded with a few extras added.