You can see the documentation, at JSON serialization and deserialization (marshalling and unmarshalling) in .NET - overview
where there is :
How to get the library
The library is built-in as part of the .NET Core 3.0 shared framework.
For other target frameworks, install the System.Text.Json NuGet package. The package supports:
.NET Standard 2.0 and later versions
.NET Framework 4.7.2 and later versions
.NET Core 2.0, 2.1, and 2.2