It varies based on the age of the video, newer ones do indeed have separate audio downloads. You can force audio only with
yt-dlp -f bestaudio <url>
This will cause the script to only consider audio-only formats, if bandwidth is a concern. However, how it decides which one is “best” is beyond me. For example, I tried one video and got a webm that contains only an audio track:
~ $ yt-dlp -f bestaudio https://www.youtube.com/watch?v=dQw4w9WgXcQ
[youtube] Extracting URL: https://www.youtube.com/watch?v=dQw4w9WgXcQ
[youtube] dQw4w9WgXcQ: Downloading webpage
[youtube] dQw4w9WgXcQ: Downloading ios player API JSON
[youtube] dQw4w9WgXcQ: Downloading android player API JSON
[youtube] dQw4w9WgXcQ: Downloading m3u8 information
[info] dQw4w9WgXcQ: Downloading 1 format(s): 251
[download] Destination: /data/data/com.termux/files/home/storage/movies/ytdl/20091025__Rick_Astley_-_Never_Gonna_Give_You_Up_Official_Music_Video.webm
[download] 100% of 3.28MiB in 00:00:00 at 6.91MiB/s
There is a small, but growing, number of retailers that have decided to apply this worldwide. Perhaps GOG is the most noteworthy. Look at anything that’s discounted there and you’ll see their “usual” price, as well as the lowest price they sold it for in the last 30 days before the current discount started. It’s a good rule, makes me more inclined to feel I’m actually getting a good deal, wish more places would do it