Strumpy-like editor windows

I’m getting more and more into tools programming for projects and i’d like to know how to create an editor window similar to what the guys have done with Strumpy Editor where you have a visual window that you can use to move elements around and wire things up.
I’m assuming that they have written an external plugin/dll which does all the work and is then rendered out in Unity?
Can anyone point me in the right direction to learn how to do something like this?


Ended up using popup windows.

Along with embedded EditorGUI functions i created draggable popup windows on an EditorWindow.