When someone is using an effect, the camera takes a video. 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 used in several different ways. For example, combine it with segmentation to separate someone from their background, manipulate pixels in the texture to modify the image, or play the texture somewhere in the scene.
Select the Camera in the Scene tab.
In the Inspector panel, click the + next to Texture Extraction.
In the Assets panel, CameraTexture will be created:
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.