The mass that is pushed backwards may be taken from the atmosphere or carried in the vehicle. Just as the wing produces lift by affecting the gas in the atmosphere, a propeller can produce thrust by pushing atmospheric gas backwards. On the other other hand, an inflated balloon that is released in a room expels gas from inside it through a nozzle, and does not require mass from the atmosphere.
Energy is required to supply the force that accelerates the aircraft.
The energy must be stored until the propulsive force is required. Chemical energy is often stored, and released in a controlled reaction at the appropriate time. On Earth, aircraft carry hydrocarbon fuel and gather oxygen from the atmosphere for combustion. Rockets that are launched into space cannot extract enough energy from the atmosphere, so they carry oxygen with them. They have both a fuel tank and an oxygen tank.