I found this by googling. It looks like there a framework for doing this - Prism Unity.
I assume Billy Hollis uses something like this - does anyone know ?
Does anyone have any experience of working with this or know of any other options to consider ?
Prism is a framework for building loosely coupled, maintainable, and testable XAML applications in WPF, and Xamarin Forms. Separate releases are available for each platform and those will be developed on independent timelines. Prism provides an implementation of a collection of design patterns that are helpful in writing well-structured and maintainable XAML applications, including MVVM, dependency injection, commands, EventAggregator, and others. Prism's core functionality is a shared code base in a Cross Compiled .NET Standard and .NET 4.5/4.8 Library. Those things that need to be platform specific are implemented in the respective libraries for the target platform. Prism also provides great integration of these patterns with the target platform. For example, Prism for Xamarin Forms allows you to use an abstraction for navigation that is unit testable, but that layers on top of the platform concepts and APIs for navigation so that you can fully leverage what the platform itself has to offer, but done in the MVVM way.
Prism 8 is a fully open source version of the Prism guidance originally produced by Microsoft patterns & practices. The core team members were all part of the P&P team that developed Prism 1 through 5, and the effort has now been turned over to the open source community to keep it alive and thriving to support the .NET community. There are thousands of companies who have adopted previous versions of Prism for WPF, Silverlight, and Windows Runtime, and we hope they will continue to move along with us as we continue to evolve and enhance the framework to keep pace with current platform capabilities and requirements.
At the current time, Prism supports WPF, Xamarin Forms and UNO. We have no plans to create new versions of the library for Silverlight, Windows 8/8.1/WP8.1 or for UWP. For those you can still use the previous releases from Microsoft p&p here and here or previous releases of this library in the GitHub Repo.