How to turn a shape into a mask?

[Digirock]

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.

[Rick Gerard]

You are looking for a Track Matte. You will find the control in the timeline panel. The Modes column contains blending modes and track matte switches. 

You will find more information if you type "track matte" in the Search Help field in the top right corner of After Effects.