Not all of the content you need for your game is intended to be created within the Editor. Most of your artistic assets will be created externally, using such tools as 3ds Max, Maya, Photoshop, ZBrush, and others. Below is a very general breakdown of the types of assets that will be created in the Editor versus those that will be created externally.
Asset Creation Locations
Created Using External Application
AI Navigation Meshes
Precalculated Light Maps