Duplicating a layer is not the same as making a copy of the source data.
Duplicating a layer
When you duplicate a layer, QGIS creates a second link to the same source file. Any edits you make to the duplicate layer are made to the source file. Any layers that link to that same source file will also have those changes.
To duplicate a layer, right click on ...
I would not class this as an advantage of using a feature dataset over other types of workspace.
When you Start Editing in ArcMap what becomes available for you to edit is everything in a single workspace. A workspace may be a folder (of shapefiles), a geodatabase or a feature dataset within a geodatabase.
This is documented in About edit sessions where ...
If snapping to an existing vertex is you problem than you want the snapping toolbar and enable vertex snapping.
There are three ways to do right angle line segments.
Use the right angle tool from the editing toolbar.
You can just use the perpandicular constraint
Or you can use the angle and length