I would expect the licensing situation to be very different for music than for normal YouTube uploads. YouTube sometimes[0][1] offers "YouTube Premium Lite" in some countries, which doesn't include YouTube Music. This heavily suggests the music licensing adds cost. Unfortunately, it's not available where I live (New Zealand), looks to still have banner ads (easily deal with by an ad blocker, but still kind of insulting to have banner ads on a paid product), and doesn't even include background play on non-music videos (not that it actually matters in my case, since I only watch on my Mac).

[0]: https://www.theverge.com/2023/9/25/23889917/youtube-premium-... [1]: https://www.reddit.com/r/youtube/comments/1d0d1sg/premium_li...