A recent video from Ship of Heroes shows the team creating mission maps.
The video,located below, shows how developers can create maps and missions or dungeons for the purpose of instance. They noted that special iconic locations and buildings are man-made environmental art, but simpler missions are produced in large quantities.
These maps are built semi-automatically with initial map generation, which allows the team to focus their technical artists on polishing and lighting. The video is about James Smith, a developer from the team who leads the team that creates mission instances..
The Ship of Heroes team was able to create their library from literally several thousand environmental art assets, in addition to their own tools. Once they have a map that they like, the work is still incomplete. It can take months to add props, Polish the lighting, and check for errors.