iCloud Storage across devices

Note that iCloud Backup and iCloud Photos are two different things. iCloud Backup only happens when the device is charging and connected to wi-fi. iCloud Photos sync dynamically as changes happen (new photos, deletions, edits) I have never noticed a significant battery impact.

If you turn on iCloud Photos on a device, the photos from that device will sync to/from iCloud and any other device using the same Apple ID if the iCloud Photos option is turned on in that device. If it is turned on in two devices, the photos on the two devices will be combined. If iCloud Photos is turned on in your MacBook Pro, photos in the Photos app on it will also be synced to iCloud and to the phones. Devices where iCloud Photos is not turned on won't be affected. See: Set up and use iCloud Photos - Apple Support

From any computer internet browser you can log into iCloud.com and go to the Photos app there and view all of the photos that are synced to iCloud Photos.

If you don't have iCloud Photos turned on in a device but do turn on iCloud Backup, the photos on that device will be backed up to iCloud. Each device has a separate backup. You cannot view or access the contents of the backup(s) except to use them to do a full restore of a device or to setup a new device. See: How to back up your iPhone, iPad, and iPod touch - Apple Support

musicspirit wrote:

I have never until recently bought iCloud storage for the iPhones because I back them up manually to computer and believed that constant automatic backing up to iCloud would drain the battery quicker.

Well, no, iCloud backup will not drain the battery quicker. It won’t drain the battery at all, because automatic iCloud backup only backs up when the phone is locked, plugged in to power, and connected to Wi-Fi.