Author here, thanks for posting this and glad people enjoyed reading. I did some editing and added a conclusion that boring QR codes are a better choice for real world use. Easier for users to understand and more reliable. These artistic designs are mostly just for fun.
One part which I barely touched was how to mathematically "force" pixels in the error correction section. It's an extension of the ideas covered in https://research.swtch.com/qart but unfortunately I don't grasp it well enough to explain properly.
>Alphanumeric mode can encode URLs, but it only supports uppercase characters, so no one uses it.
I found that they are still pretty useful because they're noticeably smaller grids. They can be enlarged to use up the same space on screen/print as a denser UTF-8 QR, thus be clearer and easier to scan. Some readers will assume that an all-uppercase URL is supposed to be lowercase and convert it, others will use them as-is. Use short URLs, like https://qr.test/thing so only the case of the path part matters, then make sure /thing and /THING go to the same place, and you're good.
One part which I barely touched was how to mathematically "force" pixels in the error correction section. It's an extension of the ideas covered in https://research.swtch.com/qart but unfortunately I don't grasp it well enough to explain properly.