How is the air we breathe different from the air we breathe out?

As everyone knows, air is a mixture of gases, and therefore when living beings inhale it, they absorb oxygen from the air from the whole mixture, and exhale everything else back. Therefore, the air that we inhale is enriched with oxygen, and the air that we exhale is already without oxygen. It is for this reason that it is advised to ventilate the premises, especially where there are a lot of people, because over time there may simply not be anything to breathe, there will be a minimum of oxygen.