Not sure I understand you correctly but designing a model is normally done in digital content creation (DCC) tools like Blender. The three.js editor is just a scene editor. So it’s not intended for geometric manipulations but for compositing the final scene based on various building blocks (e.g. 3D assets like your factory model).
You can export your model to 3D formats like glTF from Blender and then import such files into the editor.
I’m afraid I don’t understand this question, sorry.