Interested in the lesson, so that in it the object would go under another?

Question

Adobe After Effects

I have a snail crawling on a branch. I'm new and it's not clear to me how to make a branch a closing region. The snail crawls up to it and disappears under it!
I do not understand if I need to make the branch a separate layer, because now it belongs to the background.
How to properly prepare a mask or what is the name of this approach?
Thanks