A structure the occupying faction wrecks at one of your bases does not vanish. It goes up in an explosion and a column of smoke, and what is left stands where it stood, on the same ground, as a ruin. Saving and loading a campaign brings a ruin back as a ruin, and a structure you have put back comes back repaired.
All ten buildable structures work this way: the guard tower, the recruitment tent, the medical tent, the vehicle maintenance ramp, the fuel depot, the garage, the bunkers, the helipad, the barracks, and the warehouse.
A ruin is inert. There is no refuelling at a wrecked fuel depot, no recruiting from a collapsed tent, no treatment at a flattened medical tent, and no shop at a ruined garage. Nothing about the place works while it stands as wreckage.
Nothing stored in a structure is lost when it is wrecked. Containers and their contents come through the destruction untouched, and only the way in is closed. Putting the structure back opens them again with everything where you left it.
A ruin carries a repair action, held for twenty seconds, and the action itself shows the price. Repair costs a share of what the structure cost to build, and that share scales with your difficulty setting:
Because the build price itself already moves with difficulty, the two changes stack. Repair costs money and nothing else beyond any resources the structure itself demands to build. A repair beyond your means is turned down rather than left half done.
A ruin gets repaired, never dismantled and rebuilt. It keeps its place, its owner, and whatever is stored inside it.
The occupying faction does the same on ground it holds. It sends a repair detail to a base of its own where something stands wrecked, and the detail puts structures back one at a time, cheapest first, without announcing anything.
The work only goes ahead while the detail holds the base alone. A player or a player recruit within about a hundred and fifty metres of the base centre stops it. The whole wait then restarts from the top, not from where it stopped.