AMBU bag must deliver between 700 and 1000 milliliters of air to the victim's lungs, but if oxygen is provided through the tubing and if the victim's chest rises with each inhalation (indicating that...
It contains the volume of oxygen needed to supply the victim with rescue breaths of 100% oxygen.
It depends whether it is connected to an oxygen supply, if yes, it will. Otherwise no, if just on room air.
You want the maximum amount of oxygen, which is 100%, and the ambu bag, with the oxygen attached, will deliver that.
Free, but it'd help if everyone took care of the planet.