A garage is a construction and as such should obey the laws of any other construction with regards to ventilation.
One of the most important things to consider in a garage is the ventilation. If a garage wasn’t ventilated properly it would get a lot of condensation and that could lead to rotten wood and damp and decaying plaster or brickwork.
If a car is stored in the garage and the garage wasn’t ventilated properly the toxic fumes from the vehicle exhaust would stay in the garage and cause a toxic and unpleasant atmosphere and could be dangerous. It has been known for pet guinea pigs to be put in a garage during the winter and then the car was backed it, the garage locked and the next morning the guinea pig was dead, poisoned by toxic fumes.
Even if the amount of fumes were non-toxic it would still be an unpleasant place to be without adequate ventilation.
One of the simplest and best methods is to incorporate air bricks in the construction. If it is a larger garage for more than one car and equipped for car repairs an electric fan would be recommended.