As of my last knowledge update in September 2021, YouTube Music primarily used the AAC (Advanced Audio Coding) codec for streaming audio. AAC is a common audio codec known for its efficient compression and high audio quality. It's the same codec used by Spotify and many other streaming services.
Please note that streaming platforms can update their technology and codec choices over time, so it's possible that YouTube Music may have made changes or updates since my last knowledge update in 2021. To get the most current information, you may want to check YouTube Music's official documentation or announcements regarding their audio codec choices as of 2023 or later.