I'm not sure how much of that cooperation with other players is real or in fact a clever sleight of hand by the game. Sure the player names are probably real."didn't manage to unlock every upgrade or building but I don't really feel a need to go back."
That's really not the point of the game.
You help to build enough infrastructure with the help of anonymous players...
Then you build zip lines
The whole "collaborate to build" thing could easily be mostly faked.
It is clear that each player has their own unique copy of the structures, and the condition of them seems to be unique to each player.
Don't you find it odd that all structures in your own game, even those built by other players, start at 100% durability? That is clearly not the condition of those structures in other player's games... And after a while they all deteriorate to 0% unless you repair them in your game. And probably this is the same for all other players, who each have to make their own repairs in their own game.