It's the VP9 high bitrate codec, to get it you need the cookie from a premium subscribed account or yt-dlp can get it by default by emulating an iPhone header if I recall correctly. --extractor-args "youtube:player_client=default,ios" But this is done by default.

As somewhat related know that there's also a higher bitrate OPUS codec that's only available to premium cookies, it would then be the default YouTube music codec.