iPhone storage to iCloud storage

As already noted, iCloud is primarily synchronization storage. The closest it comes to behaving as archival storage is if you use optimization:

How to check the storage on your iPhone, iPad, and iPod touch - https://support.apple.com/HT201656 - "If your device is low on storage, iOS automatically optimizes its available space while installing an app, updating iOS, downloading music, recording videos, and more. To make more storage available, iOS can remove some of your items, like streamed music and videos, files in iCloud Drive, and parts of apps that aren't needed. It also removes temporary files and clears the cache on your device. iOS only removes items that can be downloaded again or that aren't needed anymore."

This still isn't "everything" but in theory is should be enough as long as you are subscribed to enough iCloud storage to meet your needs.

Buy more iCloud storage - https://support.apple.com/HT201318

You cannot do so. iCloud offers synchronization services, not archival storage. Anything you sync to iCloud will also reside in the storage of your iOS device.