In the current folder (VisualStudioItemTemplates) you should have a CSharp folder.

That folder should be copied to the inside of the folder:
C:\Program Files\Microsoft Visual Studio\18\Community\Common7\IDE\ItemTemplates

It should substitute 3 files.
C:\Program Files\Microsoft Visual Studio\18\Community\Common7\IDE\ItemTemplates\CSharp\Code\1033\Class\Class.cs
C:\Program Files\Microsoft Visual Studio\18\Community\Common7\IDE\ItemTemplates\CSharp\Code\1033\Interface\Interface.cs
C:\Program Files\Microsoft Visual Studio\18\Community\Common7\IDE\ItemTemplates\CSharp\Code\1033\WebClass\Class.cs

Now Visual Studio has proper default files.