I ran it in recovery mode (command + R), fixed the hard drive with disk utility and returned to Mojave after restart. Don't update to Big Sur!

I knew I shouldn't update the OS. Now I can't even start my iMac. It shows the apple logo and powers down again. It's a real shame for Apple to launch this buggy update. I tried holding down opt+command+p+r and it doesn't work. It's a real scandal that we have to go through this suffering every time we update the OS.