When you create a new solution in Visual Studio 2019 you must decide which framework you want to use as target.
e.g.
WPF Application
A project for creating a .NET Core WPF Application
You can choose between the installed .NET Core SDKs
e.g. .NET Core 3.1 or .NET Core 5
vs
WPF App (.NET Framework)
Windows Presentation Foundation client application
You can choose between the installed .NET Framework SDKs
e.g. .NET Framework 4 or .NET Framework 4.8
1) There is no .NET Framework 5!
2) You cannot change a project from .NET Framework to .NET Core!
3) The difference between .NET Framework and .NET core is described e.g. here
.NET Core vs .NET Framework - What’s The Difference? - InterviewBit[
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The maximum version of .NET Core you can use in Visual Studio 2019 is .NET Core 5