That is a good question. I'm not sure. I've seen a lot interface usage for testing web apps or backend services at companies when I was doing consulting work.
I personally use stubs for higher reuse, favor integration/functional tests over units, and InternalsVisibleToAttribute to allow the use of internal classes / methods from test assemblies. Its been a long while since I reached for a mocking library. I probably use structs, statics, and extension methods more than the typical dotnet dev. And for DI, I have no qualms just using concrete classes or base classes.
I also built my own extensions to xunit to enable DI in test methods, since I do more integration tests.
Extension methods for interfaces can be very powerful along with the newer default interface feature. So using that where it makes sense, I get.
In dotnet itself, interfaces are only heavily used for key extension points, so you see them more enforcing specific idioms like IComparable, IReadOnlyList or adapter/extensions for System.Data.x, Microsoft.Extensions.x, and Asp.net core MVC. For the adapter types of things there is generally a base class that implements key interfaces and you can create stubs or mocks from those.
For the files... for stuff in the root folder like .editorconfig, gitignore, etc. I generally create a virtual Solution folder and add those files to it.
For folders with larger amounts of files, I sometimes create an empty project or use specialized project for it like node or powershell project which can be installed as VS extensions, especially if its for a project where the team lives in Visual Studio.
That said, I do most of my coding in JetBrains rider or vscode these days and keep a terminal open. When I'm on a windows box, I have nano and neovim installed, so I can just quickly edit scripts that way or just do code /path/to/file as needed.