I am trying to use a mask and keep it still while a motion tracked video moves around inside it. How is this done? Im pretty new to after effects?
Apply the mask to a solid, assign the solid as matte to the layer. And if you're fresh to AE, start here:
Getting started with After Effects
Use the mask as a Track Matte.
This is very basic stuff as you rightly guessed. To make life easier for you, start here to learn After Effects. If you skip this foundational training, you will end up frustrated.
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