Why wouldn't you just google that?
Well, I've never actually thought about it. I'm assuming it's a high class filter in which the carbon atom is filtered from the carbon dioxide molecule, leaving a breathable oxygen molecule (yes, molecule, because breathable oxygen is a pair of oxygen atoms).
Mostly, they take the air and circulate it through a high-class version of a charcoal filter, but plants have contributed to it somewhat.
They use oxygen scrubbers and a system separates waste water into Oxygen and Hydrogen.
dude , i have like no idea :D