So I use to mess around with the unreal engine back when UT 2k4 was released. The way we created dungeons was to remove various blocks in the engine itself. Is that no longer an option?
I think you talking about BSP and subtracting. You can still do that if you want to, thought I don’t recommend it. You just make a large BSP and then just substract other BSP in it.
Your level geometry no longer needs to seal to be valid, so there’s no need to fall back to this technique. Brushes still exist and can be added and subtracted but instead of carving out an infinite solid you can drop a large block like McGreed suggested.
The concept of skyboxes has changed too, now they’re actual distant geometry rather than a portal texture. If you see a mountain in the background, there’s a mountain-sized terrain in the distance. For this reason it’s best to just treat the game engine like you would a 3d modeller scene.