how do qr codes & barcodes work?

The letter A has an ascii value of 65. In 8bit-binary, thats: 01000001

When a smartphone scans a QR code, the three big dots are used to align the code with a grid, then each grid cell is detected as either dark or light. This gives a binary sequence.

QR codes use a specific conversion algorithm to turn a bunch of text into a binary sequence and back.

A barcode works similarly; the distance and width of the lines encode data which can be turned back into a number.

To create the code, you simply use the official algorithm, feed the original message/number and get your code.

You asked, "how do these lines and blocks have data in them, "

What do you think "data" is?

Computers are binary. Some say that's ones and zeros, but you could also say it's black and white. Those patterns ARE data.

The letters you are reading right now are made of dots.

Because it has a standard in the variation of the lines or points in qr that represent a number or sequence, so every possible variation represent anything, and a software can use this variation to register something in their database.