All the planes you have obtained can be found in the progression tree, accessible via the Planes menu located in the bottom-right corner of the Hangar screen. You can freely swap planes between hangar slots by selecting a plane from the tree and choosing which one to replace at the bottom of the screen.

If you don’t currently have any free hangar slots, newly unlocked aircraft will still be added to your collection. However, you’ll need to manually swap them in by replacing one of the active planes in your hangar.