"C# Dev Kit" extension is only for "Visual Studio Code", not Visual Studio. It add support for building, and debugging.net core c# applications. The type of applications it supports depends on the dotnet workloads installed (not to be confused with visual studio workloads).
https://code.visualstudio.com/download
Visual Studio uses workloads rather than extensions for framework support. The workload installs templates used to create project along with sdk support.
the NET desktop development workloads adds support for .net 4.*, C# 7.3 and windows desktop app support. It does not support .net core (.net 6/7/8/9)
you don't say which dotnet runtime or sdk you want to use, so we can not tell you the proper workload.