Ukramedia.com’s Sergei Prokhnevskiy shows how you can apply a single mask to multiple layers in After Effects.
Using the set matte effect, you can easily tell multiple layers to use a vector object sitting in the layers list.
how to apply a single mask to many layer in After Effects without Track Mattes or Blending Modes
This makes it easy to have all of the layers with the Set Matte effect use one mask, in this case, a vector shape, that is hidden from Ae’s view.
You can think of set matte as a version of a track matte that can extend to other layers as well.
There are a few issues with this technique that that will arise. Depending on the type of media you are using for your layers, the issue will be slightly different. Sergei does offer some suggestions that will fix the issues fairly easily.