As we all know .NET MAUI is an evolution of Xamarin.Forms. The underlying architecture transitioned from Renderers to Handlers. The project structure is unified. But for the projects that are getting migrated and developers coming from Xamarin.Forms background would feel at home with the classic multi-project solution structure.
To effectively support this, the All-in-One project templates introduced an option to abstract the Shared components as a separate .NET MAUI Class Library.