How do I buy iCloud storage ?

Purchasing iCloud storage does not increase the phone's storage size. The iphone storage size is fixed and cannot be increased.

If you need more room, you will need to delete something such as videos, pictures, apps, etc.

The difference between phone storage and iCloud storage -

What's the difference between device storage and iCloud storage? - Apple Support

The general rule is that when you synchronize data through iCloud, that data is going to take up space BOTH on Apple's servers (in iCloud), and in local storage on each of the devices to which you synchronize it.

Photos and videos are a special case.

If you have a lot of photos and videos, using iCloud Photos and the "Optimize iPhone Storage" feature might be useful. This lets the phone substitute low-quality copies of photos and videos for full-size ones. The idea is that iCloud still has full-size copies that the phone can download those on demand later.

However, the phone gets to decide when to free up local storage in this way. If you are using iCloud Photos, and you delete a photo anywhere, that's taken as a request to delete it everywhere, including from iCloud.