When someone is using an effect, the camera is taking a live video of the person using it.
You can play this video back live as a texture in the scene. Everything the camera detects will be extracted, with the aspect ratio of the device.
The camera texture can be combined with segmentation to separate someone from their background. You can also manipulate the pixels in the texture to modify the image - for example by changing the color of the pixels.
Select the Camera in the Scene Panel.
In the Inspector, click the + next to Texture Extraction.
In the Assets Panel, you'll see a texture called cameraTexture0:
You can now apply this texture to a material.
In the example below, we've extracted the camera texture and applied it to a material, which we've applied to a plane.
A live video of what the camera can see - the face of the user - is playing as a texture on the plane.