Calling macOS experts in here 😀

iMac 21.5" 2017 - Big Sur 11.1 reboots back into Catalina 10.15.7
(This machine is not my own and it is in a relatives house.)

Upgrading via System Preferences it says 3 hrs for 12Gb, I go away for an hour and come back and it is installing with white progress bar at approx 95% complete with no estimate to completion. I go away and come back 15 min's later and it has rebooted and all looks normal except 'About this Mac' says I still have Catalina 10.15.7 on board and System Preferences says there is the Big Sur 11.1 awaiting download and install. Did this twice with same result. There is no Big Sur install icon in dock or applications folder.

Should I try any or all of the following:

 

1. Shut off sleep mode
2. Install from Safe Mode
3. Reset SMC & PRAM
4. How do I find a partial Big Sur install folder/file if any
5. If found should I delete and have another go
6. Run 3rd party app Onyx to clean up any flotsam and jetsam and any other detritus
7. Or should I wait for Big Sur 11.2

Thank you for your feed back

PS: also posted into the Apple Discussion forums but I thought I would give our own forum experts ago as well.