“an impressive display of Valve’s physics engine” is debatable, i suspect there is nothing special happening there.

Either the whole thing is just animated, and the animation is triggered when a specific barrel explodes, or in case there actually is physics attached to the girder and the gates, they just remove the movement constraints in a scripted manner. Explosion of the barrel would then be scripted to remove the constraint of a girder near it, and girder coming in proximity of the gates triggers the removal of constraints on gates’ hinges and between the gates, and a bit later the smoke.

People who have been actually involved in HL2 modding might know more, but those are just compromises between being simple and being robust i would do if i was designing it, anything more general would just risk to fail in some manner.

As to Breen speaking in some location on the same map, it was probably the simplest way. The engine allows for portals/monitors anyway, and this was probably used. I suspect also that HL(2) can only keep one map/bsp loaded at a time, otherwise the loading screens would be absolutely unnecessary as they would be able to preload the data for 2 adjacent maps, and perhaps make the maps themselves smaller pieces of the world.

You hare faced with 100 doors. Behind one of them is a car. You asked to pick one. After you pick one, the host open 98 doors without the car behind them leaving just the one you picked and one other.

You are now asked to stay with the one you originally chose or to switch. Clearly, if you played this game lots of times, you’d almost never do well by staying with your original choice. Only 1 in 100 times will that strategy pay off.

It’s exactly the same in the 3 door situation but since the numbers are less drastic, the intuition doesn’t work so well.