How to access unity from within a c# file?

I'm a beginner to c#, and I'm trying to build a project with separation of components, and the graphics and user inputs are from unity. I understand that to access the c# file in unity, I'd have to add it to my unity project as a plugin (dll file), but how will I do the converse? Say I click a button, and that had to be sent to the c# file, and consecutively, the c# file has to cause a change in the unity interface? How is this possible with a dll? I can't quite visualize how anything is happening here. Thanks for the help