I feel like both Google and Apple has just want QR codes to go away. Some of their apps use QR codes for very specific purposes, like setups, but neither of them provide a generic QR code scanning app.
The Twitter app on Android actually has a built in generic QR scanner for whatever reason, but for most purposes I use ZXing team's Barcode Scanner app. I've been looking towards using a QR code keyboard to semi-safely let me input complex passwords in apps.
[EDIT] Another reply notes that the Apple camera app actually does scan QR codes out of the box. I haven't used an iPhone / iPod in years so made an incorrect guess.
[EDIT] Apparently some Google made Android phones do actually have some QR code support in the default camera app, but the Nexus 6P (Huawei, but under the now defunct Nexus program) certainly doesn't.