Be sure to use the following procedure when erasing the old iPhones, but wait until after you have setup the new phones if you can: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
Doing that will erase the data on your old phone without erasing anything on iCloud. Waiting until you've setup the new phones will allow you to use the Quick Start method discussed below.
You may have two types of data on iCloud -- Backups and synced data.
Data that you can view on iCloud.com is synced data; one example is iCloud Photos. Set up and use iCloud Photos - Apple Support
You cannot view the contents of Backups. you can only use the backup to restore a device or setup a new device. If you have synced data, that same data won't be included in a backup (i.e., the data isn't duplicated on iCloud).
Synced data will sync to the new phone when you turn on the same sync settings on the phone as you had on the previous phone (e.g., when you turn on iCloud Photos).
If you use the Quick Start method, it will take care of everything for you. I recently did that with two new phones and it setup the new phones exactly like the old ones. The other two options work well also. See: Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support
I had the Optimize Storage option turned on in my old phones (i.e., I did not have all full size originals) and that was duplicated on the new phone. But regardless of what you had on the old phone, you can change the Optimize setting on the new phone after it is set up.
