An ingredient called nicotine is what is so addicting. Nicotine
travels from the lungs, into the bloodstream, and directly to the
brain from there, and nicotine causes the brain to demand more of
the nicotine, causing the addiction in cigarettes.
There isn't necessarily a fake marijuana, there are legal weed
alternatives. As far as the main psychoactive ingredient, which I
assume you are asking for, it really depends on which legal buds
you are talking about. Hope this helped!