Should we keep UI Canvas in Scene?

So first, Canvas can be screen space or world space.. but second, once you get used to screen space it's really useful. You anchor UI to left/right side of screen, or centered. Once you learn correctly building UI to stretch for different resolutions, you will see the usefulness of screen space. For Overlay Canvases, you should generally have "Scale with Screen Size" selected and Expand works well but I usually use "Match Width or Height" with the Height setting because we have widescreens are common. Get used to using a pattern where you create Canvases (you should have many) with an immediate Panel child that is Stretched full width and height, then maybe another Panel below that is the one you actually want to size to show a window or popup. That way you can add a Panel that blurs the background (the entire screen) or darken the background etc. But you will also notice that if you move that stretched Panel, it stays the same size of the full screen (the Canvas) but you can still move it around. If you create your own script that set's the transform to be Vector3.zero in Awake() and add it to that Panel then you can move those Panels around and work with them in the editor conveniently without them being ontop of each other or having to toggle things on and off constantly so you can see. You should be creating many Canvas with Panels.. and not just have one Canvas with everything in it.. Unity has to redraw everything when one element changes, as well as some other reasons.. Not for every little control, but you should have a pattern where you generally create multiple top level Canvas.