How to apply Masks to the complex stock footage

To get a really believable result, you'll need to split the layer at the change to the greenScreen copy, Name the Greenscreen copy Matte, duplicate the greenscreen copy again and name it Luma Matte, duplicate the footage and name it Luma Copy, then stack up the layers like this:

1. Luma Matte
2. Luma Copy
3. Matte
4. Original footage

Then you need to add a floor, a back curtain, A 3D layer copy of the back Curtain to use as a reflection, and start adding effects as shown in this screenshot.

Layer 1 - Luma Matte, has a black and white tint plus severe curves adjustment applied to increase the contrast

Layer 2 - Luma Copy, is set to use Layer 1 as a Luma Track Matte to give you the reflections on the floor

Layer 3 - Matte, has Keylight applied to generate an alpha channel

Layer 4 - Original Footage, is set to use Layer 3 as an Inverted Alpha Matte to give you clean curtains

Layer 5 - Back Curtain Reflection is a 3D copy of Layer 7 to give you a reflection of the back curtain with the blend mode set to add

Layer 6 - Floor, is just a Shape layer with a gradient fill to give you something that looks like a floor

Layer 7 - Background Curtain Freeze Frame is a color-corrected copy of the original footage layer used as the back curtain

I've included the project file (AE 23 or AE 22) so you can see how I approached the project. Warren Heaton got you started, but just using Keylight and setting up a track matte will give you a copy of the curtains moving, but to sell almost any shot that involves keying, you almost always need a lot more layers.

(Note: I did some more fiddling with Layer 1 after I recorded the screen capture to give it a better look)