How the Highrise map in ShooterExample is made?

I would love to see the workflow for Highrise map in ShooterExample. How do you initially plan the map? Do you decide on theme first? How do you prototype the map? What tools do you use for the map? Im puzzled as how do you create bridge_trim_14(its a collection of planes). If bridge_trim_14 is created in other applications(maya), how do you get the exact position of the plane? Its like ue4 map is imported to Maya. Is it?

A detailed workflow would really help for beginner like me.