This question has probably been answered before but I've not been able to find the answer, since I'm not able to frame the question with brevity.
So the whole question is :- The worlds/levels that you see in games like Crysis, Assassin's Creed, etc, what do the designers and developers use to create them? Do they use the tools present inside the engine (whether proprietary or in-house) or a separate software that exports these worlds as an importable object for the game engine to use to load models into it, etc?
I know this is a very general question but I have been curious regarding this for some time. I have used Unity in the past and it had a terrain toolkit that could be used to make the terrain, and I'm pretty sure it had completely to do with my lack of skill but I wasn't even close to making an environment that looked like the ones you see these days.