Migrate existing UWP app to work as Xamarin APP

I have existing UWP apps that I would now like to migrate to Xamarin, so that I can run them on Android devices. What are the most viable and/or practical approaches for doing this?

In some cases I will also want to continue to run these apps on Microsoft devices. I'd prefer not to have two code-bases to maintain, but that might be an option for some apps. A couple of the apps are available on the MS Apps Store.

The UWP apps are relatively cleanly architected, with separate layers and physical projects separating the UI from data access, logic, etc.

Worse-case I can make a new (Xamarin) project and UI from scratch, but what about the other projects / layers? What are my options for those? E.g. copy and paste - retain the old code/solutions (for future UWP-based support if necessary), create copies of the old (non-UI) code/projects in the new Xamarin solution (with a new Xamarin UI).

This post How do I add Xamarin Forms to an existing UWP XAML app? talks about "multiple types of Xamarin code sharing strategies and UI strategies" but doesn't provide any details or references.

This post Adding UWP option to Xamarin cross-platform application seems to be talking about doing it the other way around: taking a Xamarin app and delivering it as UWP.

What are the most viable and/or practical approaches for doing this?

Currently, there is no tool that could directly convert a UWP app into a Xarmain App. Based on your description, you will need to create a new Xamarin app to make your app available in both Android and Windows, or even more on IOS.

You could build a Xamarin.Forms app for your scenario. Xamarin.Forms allows developers to build Xamarin.iOS, Xamarin.Android, and Windows applications from a single shared codebase. As @Jason mentioned, if your app is well architected with data, domain, and service layers, those can probably be reused in Xamarin.Forms app. And the Xamarin.Forms app will convert the code into performant native controls on each platform. So you don't need to write extra code for each platform for the same function.