Apple has introduced the scanning of QR codes into the camera application since iOS 11. I still see a lot of people that are not aware of this handy update and use another third party application for this basic functionality.
Just open your camera application and make sure to center the QR code on your screen. A notification will popup with the proper action.
Most convenient way to scan a QR code is by adding the scan QR code button to the Control Centre of your iPhone which became available since iOS 12.