The spec can be found online, but it only really details specifically how to decode various features. You can take a look at JXL-Oxide and libjxl's codebases and get a good idea, and before some idiot comes out and speaks about "muh loc" the decoder and encoder bits of libjxl really aren't that hard to read. libjxl's codebase is very well formatted, files are segregated really well into different aspects of the code stream etc.

it's not ideal, but I would rather be given libjxl's codebase over the av1 spec page. (not to mention that heif is paywalled anyways which means that avif, which is heif + av1 is paywalled too... https://www.iso.org/standard/83650.html)