MAUI runs on the .net 6 or net 7 runtime. Xamarin forms run on a custom mono runtime. So a particular app needs to be one or the other. You can not use Xarmarin forms with a Maui app, they must be upgraded to Maui forms.
Xamarin to MAUI: When setting <UseMaui>true</UseMaui> in csproj, my xaml.cs files are removed
Franklin
11
Reputation points
Within the csproj file, when setting the tag <UseMaui>true</UseMaui>, all xaml.cs files that were used with Xamarin are removed. When setting <UseMaui>false</UseMaui> all xaml.cs files are returned.
This happens when using the upgrade assistant or doing the migration manually.