App restoration on Macbook Pro - will these apps be reinstalled whenever Big Sur launches?
I would expect automatic installs of apps like Garage Band and iMovie not happening when Big Sur is released.
I would plan on these being easy reinstalls from the App Store.

The iLife and iWork apps are not part of the OS. Apple pre-installs them when you buy a new Mac which can lead people to assume they are part of the OS.  You need to download them again from the App Store if you wipe a machine and only install macOS.
This has been how it works for every release leading up to Big Sur so unless Apple makes a late change, count on signing in to the App Store to download any apps you don’t migrate from a previous backup.