This uses ffmpeg under the hood and muxes the file into a .m4a file without transcoding. Basically keeping whatever compression youtube used for the audio (which is some sort of mpeg4 compatible audio, probably depends a little bit)
This still recompressed, but it’s the best you can do using youtube as the source.
uploader (almost certainly, but theoretically you could skip this step if you encoded your video well) compresses audio
uploader uploads to youtube
youtube re-compresses the audio again (almost certainly transcoding into a different codec)