How do I use OCR in iPhone Notes?
It’s done automatically as part of the document scanning feature.
What’s New in Notes for iOS and iPadOS

What else is new

Use search to find text in documents, like receipts or bills, that you’ve scanned with the built in document scanner.
Search can also visually recognize images inside your notes such as images of a bike when you search for the word “bike.”


It’s just a built in function.  You may have to reimport the handwritten note do it’s processed by the OCR engine.