"Multiple windows" isn't a feature. It's skewomorphism, trying to replicate a messy desktop covered in sheets of paper, and easily the worst idea anyone ever had for designing computer GUIs. My favorite thing about iPadOS is that it allows for multiple apps to be on-screen at once, without them randomly obscuring each other.
The ideal UI for managing multiple sections on screen would probably be to take the Blender (the open source modeling application) and apply it to the entire OS. I'm sure there's somebody who already made it on Linux if you want to deal with figuring out how to make it run... but I'd really like to see it on Mac OS or Windows.
Microsoft has kind of lead the way here, with window snapping in Windows 7 (and they silently expanded that a bit in Windows 10) but they haven't bitten the bullet and just killed the ability to arbitrarily allow windows to overlap each other.