Ask HN: How are QR scanners written?

I have been programming web and backend stuff for over a decade but I have never done any kind of image processing. I tried googling but there is so much noise in the QR space.

What I want to know is, how does QR scanner code work? How do you go from a photo of a QR to the encoded text within, allowing for all of the factors that will get in the way like poor quality cameras, off-center photos, blurriness etc? Is there a code-first tutorial or worked example somewhere?