> MozJPEG is a patch for libjpeg-turbo. Please send pull requests to libjpeg-turbo if the changes aren't specific to newly-added MozJPEG-only compression code.
Libjxl includes a better jpeg encoder/decoder, too, called jpegli. It can be used as a drop-in replacement of mozjpeg or libjpeg(-turbo). It gives ~25 % more density in the high end and allows for 10+ bits (important for HDR use).
Apples and oranges. JPEG XL is a new codec entirely (though it allows lossless conversion from JPEG).