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)