The “sealed system volume” thing is interesting, I assume the point of it being a snapshot is to be able to change out the OS atomically while it is running.
That seems to be exactly what they are planning...
"Faster updates:
Once macOS Big Sur is installed, software updates begin in the background and finish faster than before — so it’s easier than ever to keep your Mac up to date and secure."
Not while running, but on reboot. It also makes it possible to produce the new OS image by just applying deltas to the currently-running OS, since the snapshot of the "current" OS is immutable - meaning smaller downloads for your OS updates.
