When you buy a new plane but all of your hangars are occupied, you are prompted to place it in a hangar that's appropriate for this type of plane. The plane from this chosen hangar goes to the reserve one. To get it back, you will need a free hangar that suits this type of plane.
You can also place any plane in the reserve hangar if you have space there: open your reserve hangar, tap the Use button, choose a hangar, and confirm your choice to move the plane in it.