Hi!
It can be pretty hard to get started with Fusion, so I created a quick & dirty template for you to use.
Copy the code into fusion. That's your comp.
Replace the background gradient with your image, draw the masks in the 2 polygon nodes and update the paint tool to copy parts of the background like in the video.
Some tools have an "i" next to them, meaning I added a comment in the "Comment/Info" tab of that tool.
I also added a light you can play around with. To see the effect of the light in the viewer select "Light" mode in the viewer. You can just remove it, if you don't like it.
Edit: To see the lighting and shadows in the final render, go to the Renderer3D node and under "Lighting" select "Enable Lighting" and "Enable Shadows".
Once you get used to node-based compositing, you'll love it. Nowadays I just loathe After Effects. It feels so stupid compared to Fusion.
Edit: I quickly added some particles. Shows the beauty of having a real 3D space instead of the layer-based 2.5D in AE. The particles live in true 3D space, instead of on a 2D layer.
Edit: Replaced file link.
https://mega.nz/#!zyI3kD4K!1yDhWyf9clOc08rRpOcrJ1RNePsJB7z1qM4PgurkxGg