I’m serious, they don’t. They don’t know that they don’t, but they don’t. If you grab a co-workers iPhone and they have 2500 photos on the camera roll, then you know they don’t. They’ll just keep taking photos and assume “the cloud” or whatever is backing it up. For a time, it is. That is until their backup hits the mythical 5GB mark and iCloud starts pestering them to remove data or buy more space.