How do I view old iCloud Photos across multiple devices?

If you had iCloud Photos turned on in your older devices they would have automatically synced with all of your current devices where iCloud Photos is turned on.

iCloud Photos is a syncing service that automatically shows all of your photos on all of your devices.

If you didn't have iCloud Photos turned on in the older device but did have an iCloud Backup, the only thing you can do with the Backups is use it to restore an iPhone or iPad or to setup a new iPhone or iPad. If you wanted to restore the Backup to a newer device you would first need to completely erase that newer device. You can't merge the Backup with the data currently on a device.