2014 27" iMac Photos app keeps restoring and stops at the end

The first thing you should do is drag that Photos Library to another location, preferably on an external drive, so you don't accidentally mess it up.

Where is your Photos Library located? Is it in the Pictures folder on your Internal Drive? Or is it on an external drive? If it's on an external drive, and the format of the drive is not APFS format or Mac OS Extended (Journaled) format, then stop running Photos with it immediately!  A Photos Library can sit on an incompatible drive, but running it may corrupt the database.

Are you, in fact, running macOS 11.7 Big Sur on your Mac?

Have you used any 3rd party "cleaner" apps or virus eaters or VPNs?

To narrow things down, try these steps:

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. You would need to move your Library out of your own user's folder up to the general "Users" folder. 
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon. If you use iCloud Photos, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…