Opening scene-prefab in "window" with defined size inside parent scene-"window" ?

Hi!

What is most optimal method for open one scene (possible using prefabs) in “window” with defined size (and automatically resizable depend of screen size) inside another scene in “Screen Space - Oberlay” (with automatically resizing), i.e. open virtual (or real, as WinForms) “window” inside parent “window”-scene) ?

Is there way to do the same in “World Space” mode or another (including automatical resizing bacground image to full “window” without cut image and without “free spaces” in “window” (aspect ratio can be changed)) ?

Also some another and some similar questions is on:

What is most optimal way to change transparency of these “windows” ?

Please define what “optimal” means to you.