Conclusion
On this page you will learn why the app was implemented in the way that it has been, and recommended some further reading.
Application Solution Architecture
This tutorial has used an application solution architecture that uses the MVVM pattern, and swaps the content of the main window to navigate between pages; while keeping the 'top level' view model in memory to provide application state. The pages (views) are composed in Avalonia UI user controls.
This tutorial application is targeted at the Windows platform where the main window exists - so it may appear to be an over complex approach.
However on other target platforms, there is no main window. An application has to be arranged as a series of views.
To learn more about targeting applications for iOS (Apple) platforms, see here.
To learn more about targeting applications for Android mobile devices, see here.
Further Reading
For a full list of the Avalonia UI tutorials, see here.