I had to install Google Lens first. When I tap join network, the reply is that I have to manually join the network. This is Android 9, and is an Android One phone (yes, still on Android 9 and not even a year old).

Update: Actually it works with the build-in Camera. Point to QR, it decodes it and shows SSID and password, there's a wifi icon button, and pressing that immediately connects.

Nice! BTW, the camera search icon is called Google Lens.