To be honest I haven't spent much time in Unity. Back in the day you either had to parent objects to the camera to make UI, or use a completely different UI system. It sounds like now Unity scenes can be overlaid on top of each other, is that correct?
Yes, multiple Unity scenes can be loaded at the same time overlaying each other. Physics can optionally be local to a single scene too.
You also don't need to parent objects to the camera for UI. Not sure how it used to be in the past.