JSON is a way of storing structured data, e.g. as lists, dictionaries or even potentially as objects, in text files. This is called "serialisation".
The de facto way of working with JSON in C# is using the JSON.net library, which is not written by Microsoft, but which is available using the nuget package manager.
These examples can come from any text - a string, a local text file or, more exotically, a network connection to a remote API or (potentially RESTful) webserver (linked to, somewhat riskily, without fully checking the contents).
The C# project shown in the subdirectory introduction-to-json is VS.Code (loadable in Visual Studio but uses .NET core).
As outlined on Stack Overflow, open the terminal and type dotnet add package Newtonsoft.Json.
Notice the new XML ItemGroup in the csproj file.
The C# solution shown in IntroductionToJsonFull is a Visual Studio .NET Framework solution.
As outlined on MSDN, right-click on the project > manage nuget > install Newtonsoft.Json.
Notice the new file packages.config.