I tried rebooting and status bar quickly went to 100% and again stuck...
tried to turn on verbose mode (all the ctrl keys and w, Apple key and v or l) and got nothing...
Reboot holding the option key (Startup Manager)
Returns you to software update window
Hard shut-down your MacBook
Turn on MacBook while holding option+command+R to enter recovery mode
When prompted connect to Wifi, choose your language and log into your account
It should prompt you with like six options. I first went into disc utility and ran first aid on all the internal drives (two for me). I don't think this is essential but may be a safe bet
After running first aid select the option to reinstall Big Sur, it should be already downloaded and then will just reinstall
Shut down his Mac
Hold down Option + Command + P + R for about 20 seconds
Restart his iMac.