I'm pretty sure that it defaults to best quality.

goes looking at man page

   By default, yt-dlp tries to download the best available quality if you don't  pass  any  options.   This  is  generally
   equivalent to using -f bestvideo*+bestaudio/best.  However, if multiple audiostreams is enabled (--audio-multistreams),
   the  default  format changes to -f bestvideo+bestaudio/best.  Similarly, if ffmpeg is unavailable, or if you use yt-dlp
   to stream to stdout (-o -), the default becomes -f best/bestvideo+bestaudio.

So I think that it should normally pull down the best audio unless you get into some situation where YouTube doesn't offer a format that simultaneously has the combination of highest audio quality with the highest video quality; if it has to do so to get the highest video quality then, it'll sacrifice audio quality.

EDIT: Hmm. I could have sworn that there was more text about prioritizing relative audio and video quality at one point in the man page, but I don't see anything there now. Maybe it can just always get the best audio quality, regardless of video quality, can pull 'em entirely separately.