Live text is system wide on my devices. Which is to say it seems to work in almost every Apple app from Notes to Files to Mail for images I’ve saved off of the internet or sent to me via messages or photos or scans that I have taken.
The system identifying text in a document is not OCR in the traditional way we use the term OCR which completes a one off analysis of the document at the point of scanning and saves the results as part of the document.
When the system finds text in a photo, that isn’t saved as part of the photo, I’d guess it’s done at the point the photo is opened, that’s why that information isn’t searchable either.
If the identified text is not saved within the document, it can’t be searchable.