I think Android 9 and up has QR code scanning built into the camera app, same as similarly recent vintage iOS. iOS is somewhat less problematic given that ~98% of devices are running current or one version old OSes, where the Android fleet has a huge install base who won't or can't upgrade from pre Android 9 versions. Last time I looked it was still over 40% of all Android devices.
I've side loaded LineageOS into a few old old Android devices, Galaxy S3 and S4s, but my S6Edge is still running the Android7 OS it has when Samsung abandoned it. My similar vintage 2015 iPhones 6S is running fully current iOS14 - but it is the oldest Apple device that'll run it. (To be fair, my Samsung S3 vintage iPhone 5 can't run anything newer that iOS10.3).
I think both Android and iOS have been shipping a built in QR code scanner for some years now.
Wechat had 1.17 billion users last year and has had a QR scanner built in for many years now. Given that you need the app to login to their web or desktop applications, it can be presumed that that many users have the app installed, possibly making WeChat the most popular QR code scanner app.
