How to create a QR code scanner in unity for iOS?

I do not see which unity you are talking, but I guess it is not a framework of Apple's.

You should better find a community or a supporting site of it, to get better responses sooner.

I can tell you ARKit's built in image detection did not work for me with QR codes, they are not dissimilar enough from each other.
Your best approach would probably be to pass video frames from ARKit to the Vision Framework and use that to detect QR codes, it has built in QR detection via VNDetectBarcodesRequest
Vision Docs