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>Xamarin allows you to write iOS and Android apps using C#/.NET. I don't know if these use the same UI as Universal apps (Windows 10) or something different.

Xamarin used to promote creating separate GUI layers for each OS, but now have a common GUI abstracting common items into one syntax. Not looked at it yet.

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>Telerik has something new coming out called NativeScript. As I understand it, the syntax is similar to HTML and JS, but it uses actual native UI on the device.

Nativescript IS Javascript (they offer Typescript definition files as well for those more comfortable in TS), they give access to all native routines: http://developer.telerik.com/featured/nativescript-works/

Besides offering a Webview like Cordova as a control (think Webbrowser for vfp), they have abstracted a common GUI layer, probably similar to Xamarin, as both have ties to .Net GUI. You can build in code and a Xaml-similar XML. CSS for styling as well.
http://developer.telerik.com/featured/demystifying-nativescript-layouts/


Other options in that field might be React Native and Titanium/Appcelerator.
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>You may be able to do some development in Windows, but as far as I know, all of the above require a Mac to compile for iOS.