You can roughly divide barcode reading into a "frontend" and a "backend". The backend is the most well understood (but not necessarily trivial) part: you take a binary image, with each pixel corresponding to one little square in the QR code, and decode its payload. It doesn't need computer vision. The "frontend" is the part that takes the raw image containing the barcode and tries to find the barcode, and convert the barcode it finds into a nice, clean binary image for the backend. This is a computer vision problem and you can arbitrarily fancy, including up to using the latest trends in ML vision models. However, this isn't necessarily needed in most cases; after all, barcodes are designed to be easy to read for machines. With a large, sufficiently well focused and well exposed image of a barcode you can get away with simple classical computer vision algorithms like histogram-based binarization and some heuristics to identify the spatial extent of the barcode (for example, most barcode symbologies mandate "quiet space" (blank space) to be around the barcode, and have start and stop markers; QR codes have those prominent concentric squares on the corners).
As for implementation, Zxing-cpp [1] is still maintained, and pretty good as far as open source options go. At this point I'm not sure how related it is to the original zxing, as it has gone substantial development. It has python bindings which may be easier to use.
On mobile, Google MLkit and Apple vision also have barcode reading APIs, not open source but otherwise "free" as in beer.
As for implementation, Zxing-cpp [1] is still maintained, and pretty good as far as open source options go. At this point I'm not sure how related it is to the original zxing, as it has gone substantial development. It has python bindings which may be easier to use.
On mobile, Google MLkit and Apple vision also have barcode reading APIs, not open source but otherwise "free" as in beer.
[1] https://github.com/zxing-cpp/zxing-cpp