if you use yt-dlp or youtube-dl’s -F flag you can get back a list of available formats. There are typically separately encoded audio tracks for most content on youtube, meaning you can just give it the stream id and get an m4a or webm file with no extra work.

This is what I use for my personal archive:

yt-dlp -f bestaudio/best --extract-audio --embed-metadata --embed-thumbnail --recode opus --audio-quality 0

It does recode to opus but since the bestaudio is usually in that format already, you don’t need it most of the time. You can skip the --recode opus part if you want to. I keep it because I like having a “uniform” collection. You can probably change the bestaudio/best part to simply bestaudio without losing much as well.

yt-dlp has the -F parameter to list the available stream formats. (uppercase F to list, lowercase f to select)

I use -f 251 to download the opus audio stream, and afterwards use ffmpeg -i file -c copy file.opus to copy the opus audio stream in an opus file format file (codec copy so without encoding losses).

YouTube compresses the video as soon as it’s uploaded, so it’s unlikely you can obtain any sort of lossless download. You may be able to get a .wav or similar download of a video’s audio track, but there will still be compression losses from the YouTube side.

I dunno about the qualilty but I do yt-dlp -x and it downloads and extracts just the audio portion.

I use this app for it. Even adds the correct artist, title, album and such to the file aswell. I tried several similar apps but this was by far the best.