How are 2D barcodes recognized? (QR)

I'll try to keep this question simple.
Let's say a propeller is connected to an image sensor
And should output a string when it sees a QR code.

What kind of software technique is used to find the three "eyes" of a QR code?

(I assume the positions of all of the other bits are interpolated
from the centers of the eyes.)
(I imagine that there is some kind of magic digital filter like a neural network
or a wavelet filter that finds the eyes.)
I searched and found lots of info on how QR is decoded but nothing about
how to write a program that can find the QR code in a bitmap, which practically
never aligns with the pixels of the QR code. Also interesting and mystifying is that
some QR codes have round eyes instead of square eyes, so just searching for squares
wouldn't work.