This plugin allows you to inspect the editor's scene tree itself, within the editor. It's a bit the same concept as a web browser element inspector.
You can also select Control nodes visually by pressing F12 with the mouse on top of them.
You can also right click any item within the tree to reveal a context menu with a Save branch as scene button. This button allows you to save a branch of the editor tree as a scene.
This is a regular editor plugin.
Copy the contents of addons/zylann.editor_debugger into the same folder in your project, and activate it in your project settings.