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Install a Nuget package in Visual Studio Code for Unity

I want to save my level files to .json files in the Unity assets folder. Due to the limited features of JsonUtility I want to use Json.NET. I use Linux so I can't access Visual Studio and use Visual Studio Code instead. I wanted to add the package to my Unity project and took the guide from here
Install a Nuget package in Visual Studio Code
First I used this command in the terminal
dotnet add package Newtonsoft.Json
but I got this error
error: Error while adding package 'Newtonsoft.Json' to project
'/.../myUnityProject/Assembly-CSharp.csproj'. The project does not
support adding package references through the add package command.
After that I installed the Nuget extension and installed Newtonsoft.Json#12.0.2 to my project. I was prompted to run dotnet restore to be able to use the package.
After doing so I got this error
MSBUILD : error MSB1011: Specify which project or solution file to use
because this folder contains more than one project or solution file.
So what is the correct way to install third party tools using Visual Studio Code with Unity?

Unfortunately, due to Unity3D's heavy-handed manipulation of SLN files, it is not possible to use NuGet packages within a Unity project.
In the case of Newtonsoft.Json, it is available on the Asset Store for free, you can install from there.
Other packages can be used manually by downloading the .nupkg file from the NuGet website, extracting the DLLs for .NET 4.x, and placing them in your project's Assets folder.
If the library contains native code, you might not be able to use it on every platform, however. But if you get per-platform DLLs, you can set them up in the Unity editor to only be used for the correct platforms.

NuGet package fails to install in .NET Core, but works in .Net Framework. Why?

I am working on a Visual Studio solution with two projects which I will call Red.dll and Blue.dll for the purpose of this question. Note that Red.dll depends on Blue.dll. These are both compiled in .NET Standard 2.0 with the output type of "Class Library".
I added the dll of each project to a common NuGet package called "Red" using NuGet Package Explorer. It has the following structure:
lib
- netstandard2.0
- Red.dll
- Red.xml
- Blue.dll
- Blue.xml
Here is the problem: When I attempt to install this NuGet package to a .NET Core application, I receive the following error:
Error NU1101: Unable to find package Blue.dll. No packages exist with this id in source(s): Desktop, Microsoft Visual Studio Offline Packages, nuget.org
Blue.dll is not a NuGet package. I don't know why it thinks it is.
Blue.dll is located directly beside Red.dll.
There are no external dependencies other than Newtonsoft.Json which is irrelevant.
And to reiterate, Blue.dll is NOT a NuGet package dependency. I realize if you use the "Pack" feature in Visual Studio it will create a dependency, but I created this NuGet package manually to avoid that.
But most importantly: This all works perfectly in .NET Framework! The NuGet package successfully installed, both dlls are referenced, the code builds without any errors, and the XML documentation is accessible.
Why does this work in .NET Framework but not .Net Core?
I haven't found anything else online about this particular problem.

Nuget Packages are missing in Visual Studio

I have been using xamarin studio, it works fine when I download a sample project from mvvmcross. It compiles and deploys the device perfectly.
But when I open the project in the visual studio, there are a lot of missing packages for some reasons. Is there a way of handling ? I am using Nuget 2.8 in Visual studio 2013
When I open Nuget as follows, there is a button to restore the missing packages. I clicked on it.
Then it is attempting to download the missing packages, and then it realizes there are dependencies as follows:
Then I am attempting to download the dependency manually as follows but it shows the packages has already downloaded but I am still seeing the missing packages in the solution, nothing is affected.
Therefore I am getting hundreds of errors as follows:
When I take a look at the property of missing packages, I see as follows

You should update your NuGet Package Manager extension in Visual Studio and then try restoring the packages again.
The restore error MvvmCross.Binding already has a dependency defined for MvvmCross.Core is caused by an old version of the NuGet package manager extension not recognising the newer target frameworks that the MvvmCross.Binding NuGet package is using in its group dependencies. This bug has been fixed in a newer version of the NuGet Package Manager.
If you search for the 'already has a dependency defined' error message you will find several cases of this, such as this StackOverflow question, for example:
Can not install NuGet package

Right click on the solution in solution explorer and choose 'restore nuget packages'

What's the difference between a dll and nuget package?

First of all, I apologize if this is a basic question. I tried looking this up, but for some reason, I got more confused. So, I decided to ask here. Is a dll file and a nuget package the same? Are they both just being referenced in the project?

When you add features to your project via a nuget package, you're just adding files to your project. It can be javascript files (like jQuery), DLLs that your project references (like Newtonsoft JSON), or a whole bunch of things (like Entity Framework or Owin/SignalR) -- anything really.
The advantage of using the nuget package system is that it tracks it all for you. It notifies you if your added packages received an update, it removes the files and unreferences them if you take the package off your project. It handles all of that for you, so you don't have to track the files that the nuget package added, place them in special folders, make sure they get copied in your builds, all that micromanaging stuff.

From the docs, https://www.nuget.org.
"What is NuGet? NuGet is the package manager for the Microsoft
development platform including .NET. The NuGet client tools provide
the ability to produce and consume packages. The NuGet Gallery is the
central package repository used by all package authors and consumers."
A package can contain one or more dlls in addition to other assets such as config files etc.

You can add libraries via reference into your project but you would not notice when they were updated.
NuGet is a Visual Studio extension that makes it easy to pull in not only libraries but components, and most importantly their configuration into your visual studio project. It will help you manage your packages installed on your project and it will notify you when the package has new version released.
Let's say I created my own DLL, I could add my own DLL by reference. However, it won't be available in NuGet until I package and publish it first to make it available at the NuGet Package Gallery.

How to add a nuget package to the package.config the proper way?

For my C# project I am maintaining a packages.config which includes all the dependencies my project requires. Over the time I have been copy-pasting the entries manually using a regular text editor, checking in the version number I found on the NuGet website and so on. To restore the package upon a checkout, I use nuget -o nuget-packages install packages.config which worked good so far (I am using Xamarin Studio on non-windows systems, so no VS available).
I was just realizing that messing with the packages.config in a text editor couldn't be the intended way. I know from npm and bower that a npm install --save-dev <pkg-xy> will to the job and write the package version back to the config. Is there an equivalent of this in NuGet.exe?

From the command line this not supported. You will have to edit the packages.config file manually, or write a utility to do this work for you, or extend NuGet.exe with this feature.
There is no equivalent in NuGet to NPM's npm install -save
Currently the only way to have the packages.config file automatically updated when you want to install a new NuGet package to your project is to use an IDE, such as Visual Studio or Xamarin Studio, and actually install the package. Using NuGet.exe from the command line does not have an option to add/remove entries in to/from the packages.config file when installing a new package.
NuGet.exe does have an update command which will update the package to the latest version. Whilst this would update your packages.config file it also updates your project file by adding any assembly references that the NuGet package needs.

You really shouldn't be editing packages.config. Package Restore doesn't do what you think it does. It simply downloads any missing packages that are listed in packages.config.
You might think this is what you want, but Package Restore does NOT add references to your project. It also doesn't do any of the other things the package creator had intended like running an install.ps1 script.
When installing a package, NuGet handles all of this, so your project files have added references, content, etc. This and the packages.config file is what you would commit to source control. You can leave out the actual packages folder, so you don't have to commit large binary files.
When you open the solution and build, NuGet will see that the packages are missing and will download them as if you had checked them in. The actual "install" was already done (and committed). That is all that Package Restore does: no more, no less.
If you are using Xamarin Studio, you can install NuGet by following the instructions here:
https://github.com/mrward/monodevelop-nuget-addin