How do I Use ConfigurationBuilder in .net core 3.1 Winforms?

You extension method SetBasePath is defined in Config.FileExtensions.

Can you check if you are missing a reference to: Microsoft.Extensions.Configuration.FileExtensions package.

For resolving AddJsonFile, need to add a reference to the Microsoft.Extensions.Configuration.Json package.

I tried the same code that you have posted. I could able to compile and run the app after adding the following NuGet packages.

Microsoft.Extensions.Configuration (Version 3.1.0)
Microsoft.Extensions.Configuration.Json (Version 3.1.0)

Most probably you are missing these packages in your environment.