In Arch it's packaged in libjxl as /usr/bin/{c,d}jpegli . See https://archlinux.org/packages/extra/x86_64/libjxl/ , section "Package Contents" at the bottom.

I'm currently using it via XL Converter.

https://codepoems.eu/xl-converter/

nixpkgs unstable (i.e. 24.05) has it at ${libjxl}/bin/cjpegli