iPhone storage vs iCloud storage

Delete or offload content from the iPhone. Its internal storage isn't upgradable.

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You cannot add internal storage to an iPhone. Whatever it had when it rolled off the assembly line at the factory is what it will have for life.

iCloud storage (on Apple's servers) is for backups and synchronization. For the most part, it is not a substitute for internal storage. There is a special case.

if you are using iCloud Photos, iCloud will have copies of all of your photos – and so will each iPhone, iPad, or Mac that synchronizes against iCloud. Manually deleting a photo from any device will be taken as a request to delete it everywhere – including from iCloud. However, you can turn on "Optimize iPhone Storage" (or the equivalent) on a per-device basis.

When "Optimize iPhone Storage" is on, you are giving your iPhone permission to automatically replace some of the full-size photos on your iPhone with lower-quality "space-saving" versions, on an as-needed basis. The idea is that iCloud will still have the full-size copies, allowing the phone to download full-size copies "on demand" when you do something that would benefit from having one.

I haven't filled up the local storage on my iPhone, so I can't tell you how well this works (or doesn't) in practice. But this is one way to handle a large photo and video library (short of manually importing photos and videos to a place on a computer that is outside of the iCloud Photos system, then deleting the copies on the phone).

You should also read this --> What's the difference between device storage and iCloud storage? - Apple Support (CA)

If you are inclined to move photos and videos off of your iPhone to a Mac or PC, the following Apple and Microsoft Support articles outline some ways to do it.

Transfer photos and videos from your iPhone or iPad to your Mac or PC - Apple Support

Import photos and videos from an iPhone to PC – Microsoft Support