How can I revert to a previous OS from a Big Sur update?

Here is your problem:

Failing hard drive - This computer has a hard drive that appears to be failing.

You can either have it replaced or you can get an external SSD and clone the system to the SSD and use the SSD as your boot and storage drive. The drive I would recommend is:

OWC Mercury On The Go Pro and a OWC On the Pro Go Power Adapter

In addition please follow the directions below!

The best antivirus, cleaning app, and overall maintenance app for Mac OS is Mac OS itself. All you need to do with Mac OS to keep it secure and running well is to keep it up-to-date and do periodic (about 1x per week) restarts. Other than that, leave it alone. Adding third party antivirus, cleaning, security and other types of maintenance apps to Mac OS adds no additional level of security. The only thing these apps do is have the opposite affect users want. They make Mac OS slow, unstable, generate odd behavior (much like you are experiencing) and make Mac OS appear buggy. 

Please locate the developers uninstall instructions for Norton/Symantec and follow to the letter. Then restart in Safe Mode per the directions in Use safe mode to isolate issues with your Mac and then restart normally. 

Finally there is no evidence that you back up the computer, if this is correct this is shortsighted and a 100% guarantee you will lose your data. At the very least you should be using Time Machine to back up the computer. Please read Back up your Mac with Time Machine

Just to add to what others have said..

• If you can execute a Time Machine backup to an external drive that would be the best place to start. Here's the drive I use:

https://www.amazon.com/gp/product/B07VP5X239/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

• After executing a successful backup, it may be useful to run a First Aid check with Disk Utility to see what it says about the status of your internal hard drive.

How to repair a Mac disk with Disk Utility - Apple Support

• If both of the steps above are successful, it may be worth a shot to do a clean install of macOS on your current hard drive with a USB stick, then restore your Time Machine backup afterwards.

How to create a bootable installer for macOS - Apple Support (Note: links to previous versions of macOS are available on this page)

Restore your Mac from a backup - Apple Support

• I would echo what rkaufmann87 has to say in regards to Norton.. most 3rd party antivirus programs (Norton and McAfee especially) tend to put a lot of unecessary resource hogs on your system. Your best bet is to do without it if you can.
• Hardware-wise, if your internal drive is failing your best option is to go with an external boot drive as suggested previously. It is possible to replace the internal hard drive, but you (or a service provider) would have to cut off the iMac's screen in order to get at it. I would definitely go with an SSD.

There is nothing wrong with Big Sur however you may have installed something that is creating a problem. Please follow the directions belowL

Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211

Tamphrey wrote:

How can I revert to previous OS without losing what is on my computer?

Hello, Tamphrey.

From one user to another (obviously 🙄) as @rkaufmann87 posted, the key to not losing what is on your computer is to have your data backed up. Three times.

It appears that your drive is failing so today is a good day to get started with that. Your stuff is packing its bags right now.

Please also read, https://discussions.apple.com/docs/DOC-250003583, on external SSD setup instructions and migrating data.

-Jack

Your welcome.