How do you draw on top of Unity's Interface?

I'm looking into extending the editor and want to know how to draw, say a line, on top of Unity's interface if that is even possible. I know you can create your own editor window and do stuff in there, but I can't seem to find anything in terms of rendering stuff on top of the interface (like across multiple windows or like over any part of the interface). I just want to point to things in the interface (e.g. an object in the hierarchy or the inspector).

Thanks for reading.