But why not just download the mp3 then?

From where?

I think the point here is that you can run `yt-dlp --extract-audio --audio-format mp3` instead of saving as .m4a (a lossy compression) and then covering that to .mp3 (another very different lossy compression).

Under the hood, there's probably an additional lossy conversion. I'm not sure if YouTube converts uploaded videos to specific formats but if they do, then the worst case scenario is:

- original uploaded video uses .ogg audio

- YouTube converts that to opus and puts it into a container format (wbem?)

- You download the video and extract the audio to .m4a using yt-dlp

- and then you convert that to .mp3 using ffmpeg

That's 4 consecutive lossy formats, each one throwing away different data.

Honestly the best thing to do here is use yt-dlp to download whatever format YouTube provides and use ffprobe to find out what audio format is already there. Then do one conversation if required.

I usually just extract the raw Opus audio, then run it through Picard to tag and save it in my music directory. I don't see any point in converting to MP3 these days -- Opus provides better audio quality at the same bitrate (or, equivalently, lower file sizes for the same audio quality), and pretty much all player software supports it now. I've actually been going the other way and converting most of my music library to Opus and getting rid of MP3s.

Which players only accept mp3? Maybe this was true in the early 2000s

A good use case would be when you own a car built somewhere between 2005 and 2015 that accepts CDs and usb drives but only mp3 files. Some supported AAC and ogg files without them being advertised as compatible, but some might not.

Or when you keep using an old mp3 player from the early 2000's.

Do you have examples of this that aren't 20+ years old?

Many of the little generic MP3 player modules that cost next to nothing will play MP3 (obviously) and WAV, and sometimes OGG and WMA, but AAC support is relatively rare.