Hi Eric, there is a whole heap of tongue explanation/material coming in the Ultimate Level. It is quite complex and more appropriate for extended range so is purposely omitted in the earlier levels. Basically I use the simplified line of “the lips choose the pitch/overtone, the tongue makes it sound good” approach, you may have seen that. Have you seen this video, I begin to touch on oral cavity resonance: https://www.youtube.com/watch?v=u6LEZmuyk1Q&t=2s Definitely happy to address it in the next LIVE Q+A.
Cheers,
Greg