How Do They Do It?

How does people that make these stealthy grow cabinets/closets get rid of the smell coming out the exhaust? Because if u look at ebay, some of the ones have just 2 little fans and no exhaust vent. Others have the exhaust vent but still where does the smell go. Out the exhaust yes, but the other end of exhaust goes out into room ur living in.... So how do they get the smell to go away? Because I plan on making my own

 

Thanks for the feed bro. Peace