How do I scan a QR code from an image stored on iOS?
The app QR Reader for iPhone on the App Store is quite a powerful QR code app. Check it out.

I use it infrequently, but I'm glad I have it in my toolbox.

Another reader that isn't scammy or tries to advertise / monetize your use is QRafter Pro and it's very solid over years of use.

Disclaimer: We're satisfied users of QR Reader/ QRafter for iPhone, and there are no financial or other connections between to TapMedia Ltd or QRafter.