There are browsers extensions for this. I can't recommend one because I don't use this anymore. On Android this would mean using Firefox or another browser allowing extensions. Or you can give a YouTube address to MPV with the --no-video parameter. Or use NewPipe or one of its forks and open the YouTube kink with it in audio only mode. Or use invidious, but this last option is harder and harder to use. Or yt-dlp -x to download the audio of course.