why is my storage still full after buying more

Because you are confused about what you bought. You can buy iCloud Storage, which lets you keep more data in the cloud, but that did NOT increase the memory on your iPhone as iPhone storage is fixed from the date of purchase. If you own a 32GB phone, it will ALWAYS be a 32GB phone, and so on.

If you are running out of space on your phone you'll have to delete things from the phone you don't need or use, such as photos, videos, messages, apps, email, etc. Or perhaps you are now in a situation where it's time to move up to a newer iPhone with more physical memory.