I wonder what function was used.

E_mitail

I know the motion of the arm of the video is one of many functions of live2d.
But unfortunately, I'm a beginner at live2d.
I don't even know the name of the function that embodies movement.
Can you tell beginners how to learn the name or tutorial of that feature?

akaisuiren

Sure! It's probably special poses and actions....
Here are the links that I found helpful, credit to the channel owner I'll put the links down below.

Actions
Poses