Can anyone not find videos that they deleted even when having iCloud backup?

Everyone cannot find videos that were deleted from the phone if photo sync is enabled. iCloud is a sync service, not an archive service. So anything you delete from your phone will also be deleted from iCloud. If you had iCloud Photos OFF and you backed up your iPhone to iCloud you can restore the photos by restoring the backup to your phone. But note that will replace everything on the phone with the contents of the backup, not just photos.

iCloud+ has 2 independent functions. iCloud backups, and iCloud sync. It’s confusing because they both have the same name prefix. But they have no connection with each other. Go to Settings/[your name]/iCloud. You will see a bunch of switches. When you turn on a switch that data type will sync to iCloud. If you do this on multiple devices that share an Apple ID the selected data will sync to all of those devices. 

You can also turn on iCloud Backups. This will back up your phone every night if the phone is plugged in, connected to Wi-Fi and locked.

But note that anything that you sync by the first method will be excluded from the iCloud backups.