There is no way to open a Photos Library on an older system version than the version where you last opened the library. If your photos are in iCloud Photos you can recreate your Photos Library on Big Sur by creating a new, empty library and set up iCloud Photos for this empty library. The photos and albums will sync back from iCloud and you have downgraded the library from Monterey to Big Sur. If your library is not on iCloud, it would be better to upgrade your Mac to Monterey. Otherwise you can only extract the original photos from the library package and will lose the filenames and any metadata you added, also the adjustments and albums. To extract the originals select the Photos Library in the Finder, ctrl-click it, then use the command Show Package Contents. Your photos are in the folder „originals“.Or use the last backup of your Mac while you still have been using Big Sur on the Mac.