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: https://support.apple.com/en-us/HT204264
Another way that photos may be on iCloud (if you don’t have iCloud Photo Library 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.
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.
On an iPhone or other iOS device, iCloud is designed to be viewed by using the apps on the device (e.g., the Files app for iCloud Drive, the Photos app for iCloud Photos, the Notes app for notes, etc.).
You can also use an internet browser on a computer and log into icloud.com.
Also note that while you can view synced data as described above, it is not possible to view the contents of an iCloud Backup. You can only use the backup to do a full restore of a device or to setup a new device.