There are a lot of ways a system update can fail  and FWIW, my Studio upgrade to Sonoma did not go well.(Update on the laptop went fine, however.)

On my Mac Studio, I got an incomplete install with an infinite loop of reboots:

Got to the second reboot, with white logo and thermometer and then  got stuck in reboot loop cycle at about 2/3 done.

So, I did a restart in recovery mode and tried an install from there. Same thing, but this time instead of just a loop of  reboots, I got a message that there was “no administrator “on the machine, and install could not recover.

Here’s how I got out of that:

Opened the Terminal and entering

         rm /Volumes/OSX/var/db/.AppleSetupDone

(where OSX is my main volume name, so you’ll need to replace it with your own volume name.)

NOTE:  (Do this carefully if you’re following along at home: “rm” is “remove” and you don’t want to mess that up!  Copy/paste is a good idea.

Next I rebooted back into recovery and using the “reinstall Sonoma” almost worked this time. At least it seemed to complete.

However, when I rebooted again, I found myself stuck at the 2/3 thermometer, so I powered down and unplugged all “extra” cables from the studio (USB, Thunderbolt etc) and rebooted. This time the install completed 100%.

I then reinstalled all the cables and things seem to be working OK.

HTH someone.

Tracy