Jerome
don’t think to much about the position or movement of the top and bottom lip.

I recommend to concentrate on the idea of aaaaaoooo and the feeling of milk spout.

With that movement ( horizontal) you can learn to play from nonote to low note and pitch change.

You have to learn the process and to practice the process first.

In my opinion it is not necessary to observe where someone’s lips are or how it looks like.
Important is to learn the techniques, and the feeling and repeat them.
And initialise them step by step.

That worked for me and I for my students as well.

Hajo🎺