If you really want to geek out on how the ZF 9HP works, these videos are good.
But IMO knowing why the transmission does all kinds of quirky things does not excuse the fact that it does all kinds of quirky things. And its pathological quirkiness is illustrated by the fact that in several decades of car and SUV ownership I never really thought about how an automatic transmission works before I was confronted with this abomination. And I certainly never scoured the internet looking for answers or solutions, or posted endless tomes on forums complaining about the thing or reporting on premature failure. Sadly, I'm not alone. I don't think history will be kind to this transmission design.