I got a sudden fan shut down every time I would switch on my MacBook Pro (16-inch, 2019) after I upgraded macOS from Catalina to Big Sur. Here's how I fixed it.
CMD + R- Press on reboot to enter Rescue Mode
Utilities › Terminal
kmutil trigger-panic-medic --volume-root /Volumes/Macintosh\ HD
This is the Kext extension that, apparently, was causing the issue.