Question: I need to add a Unity reference (and more) for a C# plugin. Researching this question, people wrote to go to "Project -> Edit References" or to right-click the "References" item in the directory. However, the "Edit References" tab is grayed out, if it's there at all. Additionally, there is no "References" item or folder. What should I do?

Answer: 
In the screenshots you have created a Shared Project. This project is basically a set of files and does not have the concept of references. A shared project is meant to be referenced by one or more other projects and allows a simple way to share multiple files across these projects without having to use file linking.

For a Unity plugin I would expect you would need to create a C# library project which is available under Other - .NET - Library.

[![New project dialog](https://i.stack.imgur.com/lzzhc.png)](https://i.stack.imgur.com/lzzhc.png)
Underneath your game's name on the left bar which says `Solution`, there should be a subheading that says `Assembly-CSharp`. Select that and then try and select `Project -> Edit References` again.

**EDIT 2** Shared projects cannot add references, as said [in the documentation](https://developer.xamarin.com/guides/cross-platform/application_fundamentals/shared_projects/). As said as the person underneath, the best way is not to use a shared project but a library project instead.