> Or do they just throw down several lines until they find the 1:1:3:1:1 ratio?
Essentially yes. If you are aware of how one-dimensional barcodes are read, it's the same. You would need to check multiple scanlines, but you don't need the full image analysis. It might be the case that modern QR decoders actually look at the full image, but this is hardly necessary.
Either way, incredible design, it's mind-blowing to learn more about the elegance and level of sophistication.