How do I upload photos and videos to iCloud to free up space ?

Uploading photos to iCloud won't necessarily "free up space" as you will still have to use some space on your device as well. This is where the feature 'Optimize Phone Storage' feature comes in handy to conserve space on your device.

Isn't the only way to retain highest-resolution copies of your images AND free up space on your phone to (1) Download photos to a computer and/or external hard drive and/or third party service, then (2) delete the photos from your phone?

From what I read, photos stored in iCloud are deleted when you delete the photos from your device -- so uploading photos to iCloud will definitely NOT free up space on your phone. Where does "won't necessarily 'free up space'" apply? What am I missing?

"Optimizing phone storage" means compressing images which degrades the quality of every compressed image -- wouldn't it be the very last resort to recommend to someone trying to free up space on their device?