I tried YouTube for music a year ago or so, as it was included when I tried out a paid YouTube plan to remove the ads. Apparently (at least at that point) not all videos are available in "YouTube Music", so you can't just play back any of the videos from YouTube while using YouTube Music via Carplay, instead you need to then use YouTube itself, which of course you can't use via Carplay.

After being disappointed again and again, I too moved back to collecting local files for my music instead, although bought rather than what.cd as I used in my early days. Tend to use Bandcamp mostly, they also waive their fee on purchases every first Friday of the month (https://isitbandcampfriday.com/), so collecting a bunch of things to buy and listen to each first friday of month has become a nice little ritual :)