What is the general architecture of an endless runner game?

What is the general architecture of an endless runner game?

I am confused as to how endless runner games actually work, conceptually,
as far as having a never-ending canvas. Sprite Kit is under NDA, so I am
going to use Cocos2D as my framework.
There are some tutorials on the web that are specific to other languages,
and tools, but I just need to figure out basically: If I create a scene
with a specific size, how do I create the illusion of a never-ending
background? Do I just animated backgrounds behind the scene, or do I
somehow dynamically add length to the scene, so my runner really is
running along the canvas?
Do I make sense? I just cannot grasp what the actual method these games
use is. They certainly feel like the runner sprite is moving along a
canvas, but maybe it's just that he's staying still and all the elements
are moving?