Video and audio formats, plainly
Four names cover nearly all downloadable audio, and choosing between them is easier than the naming suggests.
- M4A is a container; AAC is the codec inside it
- YouTube stores AAC and Opus — MP3 is always a conversion
- Pick MP3 only for compatibility, never for quality
MP3
Old, lossy, and universally supported. Nothing refuses to play an MP3, which is its entire remaining advantage. At 320kbps it sounds fine to almost everyone.
M4A and AAC
AAC is a more efficient codec than MP3, usually delivered in an M4A container. It sounds better at the same bitrate and is what YouTube already stores — so taking M4A is a copy, not a conversion.
Opus
More efficient again, and increasingly what platforms serve internally. Support outside browsers and modern phones is patchier, so it's less useful as a keeper format.
The practical choice
Take M4A unless something in your setup insists on MP3. You get the source quality with no re-encoding step and a slightly smaller file.