Confirm all my photos are in iCloud before removing from iPhone
How to confirm all my photos and videos are in iCloud before deleting off iPhone
iPhone 11, iOS 15
If you had enabled iCloud Photos and then if you delete Photos and Videos from the iPhone it is deleted from iCloud Photos too, and vice-versa. Also, it is deleted from all the devices that use the same Apple ID.
And if you happen to take a backup either over iTunes or iCloud after deleting Photos and Videos all the photos are deleted from the backup (iCloud/iTunes) too when you take the next backup.