Which Unity Assembly holds Project folder class types?

I am trying to reference “where” all the class definitions I create, in the Project folder, are located, so I can gather the class types for use in an Editor script. Which means, they must exist “somewhere”, or I would not be able to reference them from the Project folder using IntelliSense, and would also get errors that the type does not exist if I declared using the type.

If I can put these types into a collection, I can use them in my custom Editor window.

My research has yielded that Assemblies may be where the types are stored. Is this true? And if so, which Assembly contains the Project folder’s class-definition types.

https://stackoverflow.com/questions/56032292/which-unity-assembly-holds-project-folder-class-types/56033192#56033192

This wound up being the correct answer.