I have just updated my software to macOS Big Sur and have been getting popups saying these will damage my computer.

Norton likes to install a System Extension that is very difficult to manually remove, which is why using the Norton uninstaller is the first step. In some versions of Norton, you will find an uninstall option in the Norton app. Norton also has an uninstall tool that may work if you do not have the uninstall option in the app.

https://support.norton.com/sp/en/us/home/current/solutions/kb20080427024142EN

The easiest way to determine if the System Extension has been removed is to post the free EtreCheck report using the Additional Text option when posting.

Using EtreCheck - Apple Community

Once that System Extension has been removed, it is easy to remove the files in the screenshot that you posted just by dragging them to the trash. It may require starting in Safe Mode first so they are not currently running when you try to delete them.

Start up your Mac in safe mode - Apple Support (CA)

It is best to make sure that you have a backup. Norton wreaks havoc with your system as you have found out and whenever you have to deal with system files to remove it, there is a chance things can go wrong.

Sounds like you may have picked up some Malware or are using software that is conflicting with the system. Go to Finder > Go > Go to Folder. Paste each of these locations there and post a screenshot of the files in each folder. Don't forget the "~" character at the start of the last location:

/Library/LaunchAgents

/Library/LaunchDaemons

~/Library/LaunchAgents

Norton/Symantec is your problem. Remove with any provided uninstaller and verify that the files have been removed from those locations that you have posted. Once it has been successfully removed, you will no longer see those errors.

About those "<app> will damage your computer" messages - Apple Community has some illustrations, instructions and explanations that augment what Mac Jim ID wrote. Either one of us is adept at providing remediation instructions, should you require them.