97 degrees celsius after update to big sur

That's good. Try the following: boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and see if the problem persists. Reboot normally and check again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

How long ago did you upgrade to Big Sur. What system version were you running before this starts?

After a major upgrade there's a lot of background activity going on, like Spotlight indexing the hard drive(s), Photos running in the background to identify faces and objects, etc. That might take several days or more. You can launch Activity Monitor and check to see what operations are using the most CPU cycles. That should give you some idea of what is causing the problem.

This is what my 2017 iMac runs with Big Sur, an SSD and 3 external HDs running;

How big is your Photos library? What system did you upgrade/update from to the current Big Sur version? Large Photos libraries can go for two weeks or more working in the background identifying objects in photos and fact detection.

Have you installed or run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac?

Which Mac do you have?