How do I move iPhone storage to iCloud storage

iCloud is a syncing system, not an external 'hard drive'. If you delete them on your device, they will delete from iCloud. To increase space available, go Settings/Your Name/iCloud/Photos and select Optimize iPhone/iPad Storage. That will reduce the space required by about 90% when fully effective.

For what you want to do, you need a different cloud based solution such as Dropbox, OneDrive, or Flicker.

See these documents: Set up iCloud on an iOS device, iPadOS device, Mac, or Windows computer - Apple Support

and Set up iCloud Drive - Apple Support.

And, here is some information that should help you manage your storage.

iPhone:

How to check the storage on your iPhone, iPad, and iPod touch  https://support.apple.com/en-us/HT201656

How to free up space on an iPhone https://www.macworld.co.uk/how-to/iphone/free-space-iphone-3524712/

10 Easy Ways To Free Up A Lot Of Space On Your iPhone https://www.huffpost.com/entry/iphone-free-space-tips_n_6269756

iPhone Other storage: What is it and how do you delete it?

https://www.macworld.com/article/3388219/iphone-other-storage-what-is-it-and-how-do-you-delete-it.html 

iCloud:

Check your iCloud storage usage on iCloud.com https://support.apple.com/guide/icloud/check-your-storage-usage-mm6e5a441fc7/1.0/icloud/1.0

Manage your iCloud storage https://support.apple.com/en-us/HT204247