When they're on their phone it'll show a download button instead :-)
The QR code only appears on desktop.
Again, what do you gain with inconveniencing the user and adding friction, instead of just taking them to Play Store and letting them install with one click?
You can take a screenshot and long click the QR to go to the link. At least on iOS. No need for another phone ;)
Not exactly obvious to say the least, but it works.