I want to back up my iPhone to Google one, as I’m not going to pay for space on more!

It is not possible to backup everything in Google One that an iCloud backup stores. None of your iCloud data itself can be backed up there (notes, contacts, messages, etc). Nor any of your passwords, settings, nor a record of your apps (used when you restore an iCloud backup to reinstall those same apps from the App Store). No Apple Health data can be stored on Google One either.

You can backup photos, Google Drive files to Google One, but you cannot duplicate an iCloud backup there. So you’d be unable to fully restore your device if necessary.

If you don’t have enough iCloud space, use a windows or Mac computer and iTunes or finder. An encrypted iTunes backup is essentially identical to an iCloud backup and can be used to fully restore your device.

I have no idea if this includes everything that iCloud includes but see the following from Google support: https://support.google.com/googleone/answer/9149304?hl=en&co=GENIE.Platform%3DiOS