Each video must be subjected to postproduction process before it reaches the recipient.
The most popular one is compositing.

These activities include:
removing unwanted frame elements, manipulating colors and other elements of the filmed world to get the desired effect, but also linking video layers including greenscreen keying, or finally combining video content with generated 3D and 2D elements. This is what compositing artist does.

The following clip is an example of a compositing process done in various situations, from full 3D comps to CGI integration with the film footage.