what is the difference between these two libraries?
https://www.nuget.org/packages/Microsoft.AspNetCore.Mvc.NewtonsoftJson/ vs https://www.nuget.org/packages/Newtonsoft.Json/
what is the difference between these two libraries?
https://www.nuget.org/packages/Microsoft.AspNetCore.Mvc.NewtonsoftJson/ vs https://www.nuget.org/packages/Newtonsoft.Json/
This is an extension to Newtonsoft JSON.
See the nuget dependeny list here

It appears to connect the Mvc infrastructure of AspNetCore to newtonsofts JSON and extending it by various features.
As @Samuel, said it is just an extension of Newtonsoft JSON. If you check its source repository you will find classes from Microsoft.AspNetCore.Mvc.NewtonsoftJson project using Newtonsoft.Json.
Here are the classes from Microsoft.AspNetCore.Mvc.NewtonsoftJson source:
If you want to dig into source code then,
GitHub repository Link: aspnetcore/src/Mvc/Mvc.NewtonsoftJson/src/