I kind of understand how a UPC code works, but how is a QR code capable of “storing more data”?
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Imagine if you made a document in something like Word, and you pasted in a barcode image on one line, then hit Return and pasted a different barcode on the next line, then hit Return and pasted a different barcode on the next line, and so on. Line after line of barcode images. Now imagine if you didn’t have to make those barcodes very tall – if you could squish the images’ height down so each one is short but still wide. Now you take those lines of squished barcodes and remove the spacing between the lines of the document so they butt up against each other. Now you have a little rectangular block made of multiple rows of barcodes. That’s *sort of* what a QR code is doing (*not exactly, because there’s other blocks embedded in the middle to help with formatting and fixing alignment in case the image is bent or folded, and QR codes use a lot more redundancy for error detection and repair, so “rows of barcodes” is only *sort of* what QR is doing.*)
Now imagine if you print this document out on paper, and then cut out the square with the codes and hand that to someone to decode. One problem they’d have is that they don’t know which way was “up” on the paper in the first place because you cut it into a square. I’m sure you’ve seen that QR codes always have this big ugly box in 3 of the corners but not all 4 corners. That’s to solve this problem of which way is up on the image. It tells the computer which way to rotate the image in its head before decoding it. It should rotate it so the corner without the big box is in the lower-right.
As to how much more it can hold than a UPC code, that question isn’t simple because there’s multiple UPC code standards and multiple QR code standards. You may see numbers quoted like 7000 different digits, but keep in mind that that refers to the biggest most dense QR codes which are not usually what you’ll find in general public usage. What you typically find in consumer usage only holds a hundred or so digits. Usually that’s enough to encode a basic alphanumeric string for a web URL, as long as it’s not super super long. (An address of about 50 chars or less).
As to why not use the denser format? Well, because it’s easier to get a “good scan” when the image is clunkier and blockier. It makes it a lot more tolerant of noise. For things like labels on store products, or images quickly snapped with a mobile phone, that tolerance of noisy images is important.
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