How to restore missing Photos and videos on Mac

I have many dozens of photos and videos missing from Photo Library. Its possible the loss occurred during recent upgrade to Big Sur, but possibly before then. Some time ago, during conversation with Apple Support, I rebooted in Safe Mode and, all of a sudden, the missing photos/videos were there in PhotoLibrary. But now, again, I can't find them - in any Photo Library. Unfortunately, no backup from this time.

Strange fact: many of the missing photos (Monterey Aquarium, June 2016) are fine and visible in Photo Library/iCloud, but others, e.g. fine videos from the same time and place, have disappeared. They must be there somewhere, but I can't find them.

Any ideas how to restore or recapture these?

Many thanks in advance!