"Here you can store unused planes, and scrap them into pieces to sell or gift."
The Hangar is used to store unused planes that you remove from your fleet. New airplanes will not appear in the Hangar, only planes that have been removed.
From the Hangar, you have options to scrap or fly the airplane. Scrapping the plane will dissemble the airplane into its 3 parts (Controls, Body, and Engine), and move all 3 of those parts to the Parts section. Choosing to fly the airplane again will cost you the amount of Bux that is displayed next to the Fly button.
You are only able to send an airplane to the Hangar, removing it from the fleet, when the airplane is on the ground and has no passengers or cargo on board.