Designing a Level

Hi!

A lot of 3D games have huge levels, with great details. How do these games achieve all this? What tools do they use to create so huge levels, with such detail?

Thanks!

They sculpt them on high poly model then they bake norma mapl and ao map to the low poly version one.

Spend alot of time on textures to make them look good and have depth.
Like above, most models use Normal Maps to achieve great levels of detail
some things like grass and debris can either be handpainted or automatically generated to add more detail to the game world. Other reusable assets are used in many places to add consistent detail throughout all of the level. Just my thoughts.

Are there any good links that show how this is done? Do you have to recreate a low-poly model with the same dimensions and stuff placed in the same spot. Or do you simply make a low poly model first, save it as a different file before you add the awesome levels of detail?

You start with a diagram. You build a 3d stage, low poly, from this. Mock up as much as possible in low poly, this helps you assure the scale is correct. Then, you take those elements, and you sculpt/model in the high res detail. There are a few videos showing elements of this on the web, but you probably wont find any complete break downs of the entire process. Search for modeling game ready assets, and that might give you an idea of what it involves and where to start.

Here you go.
http://cg.tutsplus.com/tutorials/blender/create-a-lifelike-video-game-terrain-in-blender-day1/