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JPEG-XL can be a quick win since it is possible to do a transcode of all of your existing ('classic') JPEG files with zero (visual) losses in quality.

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I always think of transcode and lossless as opposites. How is this achieved in this format?

It extracts the already quantized DCT coefficients and related data from the JPEG bitstream and re-encodes them with JPEG XL’s more efficient entropy coding and container,

Think of it as zip compression that is specially designed for JPEG and putting the JPEG data in a different/better container.

If you're transcoding why wont you go to avif or another one? If jpeg -> jpeg-xl uniquely better?

JPEG is a lossy format. JPEG -> AVIF adds a second loss operation. JPEG to JPEG-XL is lossless but with a smaller file size. So one does not further degrade the jpegs beyond what they were already degraded in creating them in the first place.

It's a lossless conversion while still saving spaces. You can even convert back to get the exact same file bit-perfect

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