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.

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.

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.

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).

If you want an online tool, cobalt.tools can rip the original youtube audio if you choose "best" format in settings -> 🎶. It's also FOSS with no tracker/ad garbage so I can recommend it as an alternative for yt-dlp which has been mentioned already.

I believe Jdownloader 2 lets you do link grabber with YouTube links that can download audio only with a choice of format.

Get yt-dlp then run: yt-dlp -x 'video-url'

I believe if you're willing to check the format codes on the video you can download audio only but both will get you the least compressed audio available.

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.

Easiest way is to use an Indivious instance, if you are on desktop or use the Newpipe App if on Android.

Not a direct answer but for a while I had iTunes Match as a companion service which could be an imperfect workaround to get your hands on 256 Kbps AAC versions. (if it matches, which can’t be manually done)

I bailed when it glitched on a couple songs and syncing broke for a while. Direct iTunes sync hasn’t let me down since but it was nice while it lasted and the Matched songs I had are still kicking.

Of course it requires using iTunes and paying for that $25 for a non-guaranteed quality boost.