When everything works as expected in iCloud Photo Library, you not only have access to all your images and videos on every device logged into the same iCloud account, but they’re all backed up by Apple, too.
However, disorder can enter this perfect realm unintentionally. Macworld reader Jack and his girlfriend—let’s call her “Jill”—tripped into a situation when she logged into his Apple ID to install an app he’d purchased. (Sure, technically, that’s not how Apple wants you to share apps. It would rather people use Family Sharing, but that feature remains weak and inconsistent, and I understand when people are driven to use this alternate approach.)