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Native (C#, C++) UI plugins for Cordova Windows?

For Cordova iOS and Cordova Android it is possible to create plugins that create native UI on top of your app - you just include the libraries in your plugin and execute their API. Example for Android InAppBrowser.
For Cordova Windows, the native code is HTML and Javascript, hence the InAppBrowser plugin also adds a standard HTML tag for a Webview (or even plain Iframe) to get the same effect.
But you can also create plugins for Cordova Windows that use C# or C++ native code via a Windows Runtime Component. A public example is the globalization plugin that uses a .winmd file to offer APIs to the Javascript code (source code for this). This works great and is pretty awesome.
Unfortunately all the plugins that I could find use this C#/C++ native plugin mechanism only to receive some method call, do something with the parameters and return some data. None of those open any windows or create any other GUI (that might for example be offered by an external C# SDK).
How can one create a plugin for Cordova Windows that creates User Interface with native (C#, C++) code? Is this possible at all?

Got a response in a Apache Cordova mailing list that I am reproducing here:
Unfortunately, No. C#/C++ Portable Class Library ( PCL ) code cannot
render on top of the web component.
The libraries that you can use are limited in PCLs and for projects
targeting Windows Store it is not possible to render UI.
If the entire cordova-windows platform were re-architected to be a C# or
C++ based application, with a native webview container in which to render,
then everything is possible. This is a long risky road though ...
everything from the cordova.js and the bridge would have to change, and
every plugin would need to be rewritten.

Common controls for UWP and WPF

Is it possible to create common controls that can be used for both UWP and WPF applications?
Tried digging over the .NET and found out that code can be shared across them using .NET Standard but what about XAML controls/UI ?

You can't directly use WPF controls in UWP apps, as there are many features of WPF XAML which are not yet available in UWP. So direct forward compatibility is not available here. Evenso, upgrading existing WPF controls to UWP should not be too hard, usually all it takes is to rewrite some parts of XAML but the actual business logic behind them can be ported 1:1, as most is covered by .NET Standard.
However, at Build 2018 Microsoft announced UWP XAML Islands functionality, that will allow developers to upgrade the UI of WPF apps to modern UWP design, including Fluent Design System features. In the near future you will be able to do this with all UWP controls (including custom), once the APIs are ready, but currently you can at least use the modern UWP WebView control in WPF, which is part of the Windows Community Toolkit.

Embedding Xamarin Forms or native embedding

I will make an app where I need the native map. Either I do the app with Xamarin native and Embedding Xamarin.Forms in Xamarin Native on all pages without the page with the map. Or if it's possible I'll do an Xamarin Forms app and embedding the native map control, if it's possible? I read Adding Platform-Specific Controls to a Xamarin.Forms Layout but is the map included? Or is it only "small" controls as the label?
What should I use?

Well Native Embedding is hard to implement ( you basically need to use the shared project instead of PCL), therefore for many it is easier to user CustomRenderers.
Xamarin.Forms Embedding in the otherhand, is still preview looks like easy to use , but to be honest I have no idea what advantages it gives us. Like if we are making a native app why do we need to have a xamarin.forms loaded to our projects and not only it will make our app bigger but it will make it harder to follow.
The way I see it from your question is to use the maps given by Xamarin.Forms here and you can access it's custom renderer if it is necessary. Or use TKCustomMap which they have already extended the map

I was helping in a project when developer did the follow and it seems worked for him. He did Xamarin Forms, created
public class CustomMap : Map
where Map is Xamarin.Forms.Maps.Map. Then created custom renderers
[assembly: ExportRenderer(typeof(CustomMap), typeof(CustomMapRenderer))]
namespace CustomRenderer.iOS
{
public class CustomMapRenderer : MapRenderer
and
[assembly:ExportRenderer (typeof(CustomMap), typeof(CustomMapRenderer))]
namespace CustomRenderer.Droid
{
public class CustomMapRenderer : MapRenderer, GoogleMap.IInfoWindowAdapter, IOnMapReadyCallback
Then you can do a lot of things with CustomPins, InfoWindow, etc

You can embed any native control in a xamarin forms app. This is a feature already released while the xamarin.forms embedding into a native app is still in preview.
I would either embed the native map as you suggested or do a custom renderer for the map control already available in forms.

Creating UI in DLL and use it in external apps

Sorry, I don't have much experience in programming and English and my question might be awkward. It relates to Windows 10 applications for classic desktop and touch screens.
I want to add a nice-looking touch-oriented Modern-style UI to my non-Visual Studio application. It is possible to call an external .NET assembly from my app and use its public constructors, methods, properties, and handle its events.
So I’ve created a new WPF User Control Library project in Visual Studio, replaced default User Control with a new Window, and built a dll file. Now I can use it in my app as described above. Everything seems to work fine.
Here are the questions:
Am I doing it right? Is building a dll from WPF project the only way
to make a UI dll that can be used in external applications?
How can I make the UI looks and acts like Windows Universal Apps (at least I want it to respond screen rotation)? As I understand I
cannot build a dll from Universal App projects and I have to create
WPF window, make it full screen, and add appropriate UI elements.
Is it possible to use Windows 10 contracts and extensions in such UI
dll?

If you want to share your code between different programs, you have to make it a shared library which is dll in windows. I don't know the other two questions.

Can I use WPF's Full-screen UI on Xamarin?

I'm thinking about starting to use Xamarin, is it possible to take a full-screen WPF project and use it on iOS and Android ?

No. Xamarin.Android and Xamarin.iOS expose the UI of the relevant platforms - you can't just use WPF on them.
However, if you separate out your "business logic" from the UI logic in your application, you may well be able to use the same business logic project, and just create new UI projects (one per platform). Of course, that relies on your "business logic" only requiring functionality which is available on Xamarin; more generally you'd want to create a Portable Class Library with that code in.

No this is not possible. You have to use the Native API's for User Interface on both IOS and Android when you use Xamarin.
You can definitely share a lot of code, but WPF will not work.

No. Xamarin relies on each platform's native UI stack. You cannot take a WPF UI (or a Winforms or Silverlight UI) and run it on iOS or Android. What you CAN do is take the underlying core of your WPF project (domain, services, data, business logic) and use Xamarin to create an iOS and/or Android native UI on top of it.

WPF is only supported on Windows - it's not even supported on other platforms with desktop APIs for Mono.
In general, with Xamarin, the approach is you share "logic" code, but use native UI projects for each platform.