I’ve lost 2000+ photos and videos after iPhone 15 was repaired at Apple

You appear to be misunderstanding iCloud Photos and iCloud Backup are two separate features. If iCloud Photos is enabled it is for syncing and the photos are then not included in iCloud or iTunes backups.

With that info, was iCloud Photos turned on recently but going to the appointment?

If you use a browser and go to iCloud.com/photos how many photos do you see?

Backups do it include items already synced to iCloud. If you have iCloud Photos enabled, those items sync to iCloud when you take the photo and would remain in iCloud unless acted upon. You can verify the iCloud status in photos. Open photos, tap library, scroll to the bottom for the item count and iCloud status.

What was the status showing with the photo count at the location detailed by muguy?

The confusing part was that you were saying that your phone backs up every night and your photos were available in previous backups, but you are using iCloud Photos. When iCloud Photos is being used, then none of those photos is saved in your backups and they are only available when they are synced after entering your Apple ID. They will not be restored from a backup.

Is there a chance that your iCloud storage is full if you are using the free 5GB of storage? You won't get any errors when taking pictures if your storage is full, but it will be noted in the sync status on the bottom of your Photos app. You also get an email sent to the Apple ID email address saying your storage is full, but some users no longer have access to their Apple ID email and never receive these emails.

Were you using iCloud Photos? If so, any photos that were synced are shown at iCloud.com/photos. The files may take some time to sync.

If you were not, photos are part of your device backup and would be replaced as part of setting up your device From your backup.

Unfortunately they are gone. muguy saying they take time to sync to iCloud Photos means if they had not finished syncing before the device was erased for service they were never saved or backed up and cannot be retrieved.