How does the dr fix a collapsed lung?

Find out the cause: Need to find the cause first for the collapse-pleural effusion, pneumothorax, endobronchial tumor--etc. Appropriate intervention depending on the cause.

It depends: A large penumothorax from any cause is treated with a chest tube placement or at least evacuation of the air outside the lung. A small one can be treated with 100% o2 by mask and observation. If it's from a ventilator, even a small size may need chest tube. Pleurodesis or sticking the lung to the chest wall is considered depending on the cause of the lung collapse.