iCloud won’t back up plenty of storage

You can turn on or off items that sync to iCloud. That is not the same as iCloud backup. You can also choose to not backup data from 3rd party apps: Go to Settings/[your name]/iCloud/iCloud Backup and tap on your device name. This will tell you the current and next backup size, and list apps that are optional in a backup. You will note that iOS itself, its settings, and built-in apps are not listed and cannot be excluded from a backup. Compare the next backup size with the available storage from Settings/[your name]/iCloud.

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 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. 

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 anything that you sync by the first method will be excluded from the iCloud backups.

How can a backup be only 1GB? Go to Settings/General/iPhone Storage. At the top what does it say for amount of storage used? Theoretically the backup can be that size or larger (accounting for overhead). In practice it won’t be, because that figure includes iOS and app code, but it still includes all data on your phone. Do you mean your phone has only 1 GB of data.

Then go to Settings/[your name]/iCloud. How much storage is available?

An iCloud backup is all or nothing. You can’t limit it to a specific size or specific data. It will back up all data on your iPhone.

I repeat:

You will note that iOS itself, its settings, and built-in apps are not listed and cannot be excluded from a backup.