How can I buy more iPhone storage

You cannot add storage to the iPhone. If you need more space, you'll need to remove pictures/videos/apps/data or purchase a iPhone with more storage.

No, the maximum your phone can have is 64GB. They sell iPhones with storage up to 1 Terabyte.

No, it varies by phone. My phone has 256GB of space. The 13 Pro goes up to 1 TB

That is correct.

You're welcome.