iOS Device Storage vs iCloud

It is not possible to buy more storage for an iPhone or iPad; it will never have more than it did when you bought it. What you can buy (actually “rent”) is iCloud+ storage that is used for backing up and also for syncing to other devices, not to add to the device’s internal storage. Here is an explanation→What's the difference between device storage and iCloud storage? - Apple Support

iCloud+ has 2 independent functions. iCloud backups, and iCloud sync. It’s confusing because they both have the same name prefix. But they have no connection with each other. Go to Settings/[your name]/iCloud. You will see a bunch of switches (with iOS 16 or later also tap Show All). When you turn on a switch that data type will sync to iCloud. If you do this on multiple devices that share an Apple ID the selected data will sync to all of those devices as if by magic. 

You can also turn on iCloud Backups. This will back up your phone every night if the phone is plugged in, connected to Wi-Fi and locked.

But note that these are independent functions; anything that you sync by the first method will be excluded from the iCloud backups because it would be redundant and use iCloud storage unnecessarily.

You get other benefits with iCloud+

• Hide my Email, which lets you access websites without giving away your primary email account (and also track who they sell your identity to)
• Private Relay, which is sort of a super VPN (at much lower cost than commercial VPN)
• Custom email domain, so you can create a personalized iCloud email address that forwards to your Apple ID email (such as jane@doe.com)