How to restore lost QR Reader App on my iPhone?

Find it in the App Store and download it again.

M1 Mac mini, macOS 14.4.1;  Watch, series 7 GPS/cellular, watchOS 10.4; iPhone 12 Pro, iOS 17.4.1; TV 4th Gen, tvOS 17.4; HomePods, audioOS 17.4 and Airport Express w/ Airplay 2

How can I restore my qr reader app I SOMEHOW HAVE DELETED IT seems

You stated in plain English that you deleted it. That means it's gone. That it's no longer on your phone.

If in fact, the app IS on your phone and has stopped functioning, the first step is to delete the corrupted app and download it again.