iOS 12’s Camera app can scan a QR code. All you do is launch the Camera app and aim it at a code, like this:
You will see a little notification banner, like this:
Tap the notification and presto, you’re done. Neat.
Note: not too long ago the Camera app didn’t work this way. If you aim your iPhone’s camera at a QR code and nothing happens, your iPhone might be on an older iOS. In that case, consider updating. If you don’t want to update your iPhone’s system, or can’t, you can use the Qrafter app to scan QR codes instead of the Camera app. It’s free, and it’s great.