The indestructable building in town isnt actually part of the town object group. Its the end point of the supply route reaching the town. Like those small buildings with adjacent metal tower out in the weeds where the roads start, they are not destroyable (follow a road to find one).
The indestructible Hangar on field is the one where you select plane and loadouts.... its the one you go to when clicking HANGAR on your clipboard. If it was destroyed, you couldnt go there at all....