System.Text.Json

Added in .NET 5.0 (or Core 3.0).
The System.Text.Json library is included in the .NET Core 3.0 shared framework, while for other target frameworks, developers need to install the System.Text.Json NuGet package.

link - visualstudiomagazine.com/articles/2020/07/28/json-serializers.aspx 
link - learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to 

JsonSerializer

A class to serialize objects or value types to JSON and to deserialize JSON into objects or value types.

Method - Serialize

System.Text.Json.JsonSerializer
Provides functionality to serialize objects or value types to JSON and to deserialize JSON into objects or value types.

Method - Deserialize

System.Text.Json.JsonSerializer
Converts a JsonDocument representing a single JSON value into a returnType.

JsonDocument

A class for examining the structural content of a JSON value without automatically instantiating data values.