In iPhones (iOS 11 or better) and certain Android phones from Xiaomi and Motorola you just take a photo and it will detect a possible QR code and open a browser if there’s a link. The way it should work.
In Android phones you can otherwise use Google Lens and Google Now! to read QR codes.
If they don’t work the way you want we recommend CliqTags QR Code Reader, as it’s very simple and to the point.
You can also use any of the many free QR/bar code readers available for smartphones.