Smoke moves up and out and fills a room from the top down.
It therefore reaches the floor last, which means it cannot easily get under the door. This is why we are taught to stay on the floor in case of a fire.

Fire needs oxygen, fuel and heat to survive.
So if a fire can be compartmentalized, its spread will be slowed because it lacks oxygen. If the oxygen supply is completely cut off, the fire may even die out.

The more you can control the oxygen, the more you can control the fire.
If you close your bedroom door at night and a fire breaks out in the kitchen while you’re sleeping, you may have time to wake up and figure it out. That might give you just enough time to leave the building, or to call 911. But it could also give you a safe place to wait for the firefighters while the smoke, heat and toxic fumes are right outside your door.

You should actually close as many doors as possible in your home at night.