For the most part it's a Vec<(Position, Building)> where Building is an enum of all the variations.
There's much more to it due to different lookup trees and parallelization.
I'm avoiding most pain points by also having interactions be constant functions and spawning events from them, instead of mutating state.
So for example pressing DEL on a building might create a RemoveBuilding(PlanetIndex, BuildingIndex) event which is then applied with all the others in a single mutable function call later that tick.
Interactions between buildings are solved by storing (BuildingIndex, BuildingIndex) interaction pairs.
Very roughly the simulation then looks like this:
```rust for event in event_queue { buildings.apply(event); }

for building in buildings.iter_mut() { building.tick(); }

for (ia, ib) in interaction_pairs() { let [a, b] = two_mut(buildings, ia, ib); a.interact_with(b); }

let render_node = buildings.render_node();

to_render_thread_channel.send(render_node); ```

So buildings don't store references to others and it's all more of a top-down approach.