I have multiple devices (2x ipads, 3x iphones, etc.) and they are all under one apple ID. If I upgrade my iCloud storage and backup each device, will I be able to view all of the iCloud photos on each device (or at least have access to them)?
iPhone XR, iOS 14
If you turn on iCloud Photos on each device (note that is not the same thing as iCloud Backup) all photos will sync to iCloud and with all other devices. Note that the original sync may take a long time (e.g., days) depending on the number of photos. All of your photos will be on all of your devices and any additions, changes or deletions will automatically sync with iCloud and all devices. If you need to reduce the storage used for photos on any of the devices, you can turn on the Optimize Storage option on that device. See: Set up and use iCloud Photos - Apple Support
If you turn on iCloud Photos on each device (note that is not the same thing as iCloud Backup) all photos will sync to iCloud and with all other devices. Note that the original sync may take a long time (e.g., days) depending on the number of photos. All of your photos will be on all of your devices and any additions, changes or deletions will automatically sync with iCloud and all devices. If you need to reduce the storage used for photos on any of the devices, you can turn on the Optimize Storage option on that device. See: Set up and use iCloud Photos - Apple Support
iCloud Photos - Multiple Devices
