How to turn a shape into a mask?

I've started creating masks circa 2005 on the Flash software (today called Adobe Animate).

There, you could simply create a shape layer above a footage (or vector layer, etc) and then right click and turn this shape into a mask. Voi-la: the layer below this shape mask would be affected by the layer above.

I've been searching all over the web a way to do this on AE. 
Couldn't find it! All the tutorials only shows how to create masks on the SAME layer of my footage.
I don't want this. Why not? Cause I need to RESIZE the shape layer (i.e: the mask). And: this shape mask was made with Bezier, it has lots of details.
So...I'd truly appreciate any help here.