You can put the mask directly on the video layer or use a shape later or mask a solid and use that for a track matte. To learn how to use masks type "mask" in the search help field at the top right corner of After Effects. To learn how to use truck mattes type "track matte" in the same search field.
Yes but how to invert or subtract the mask from the composition? I use the pen tool around the video below the new created solid and the inside is masked or cut out. I do not see a subtract tab anywhere, which is the answer from many many tutorials. I need the outside of the pen tool shape masked out and green screened over. Thanks for sharing your information please.
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create a mask and make sure you switches or modes column is visible. type M and you will see the mask mode. you can set it to subtract or click invert
you should read here about it: