In some cars and vehicles the airbag will still deploy even when nobody is in the seat where the airbags are to protect the passenger.
But some vehicles will still have all airbags deploy in the event of a crash serious enough to cause the airbags to deploy.
That is if the switch to the airbag has not been shut off for the passenger seat.
But in some vehicles only the airbag will deploy for the people sitting in the seat when the seat senses someone in the seat.
Some newer vehicles have sensors in the seats that tell the computer system if someone is in that seat and then the airbags will deploy if they are needed.