You can backup an iPhone to iCloud or to a computer. See: How to back up your iPhone, iPad, and iPod touch - Apple Support
You can also sync data with iCloud. Syncing is dynamic (happens automatically as data changes) while backups are periodic. Synced data is not included in Backup (i.e., the storage isn't duplicated).
You can view most synced data at www.icloud.com but you cannot view iCloud Backup contents.
iCloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See: Set up and use iCloud Photos - Apple Support
Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.
Another way that photos may be on iCloud (if you don’t have iCloud Photos turned on) is via a backup of an iOS device. The only thing you can do with a backup is use it to restore a device or set up a new device. The contents of backups are not viewable at iCloud.com and if you delete a photo from a device it will not be included in future backups and will be lost.