It was all done in existing locations with existing assets. Areas can be roughed out in a matter of hours, play tested for a few dozen and then you go back and polish the rough geometry blocking with those existing assets and set up lighting, fine tune things, play test more (this goes on for a week or two) and you're done. 9 weeks with the amount of bugs in it is actually quite believable. The team only had 9 weeks to make TDB because Jones stalled the base game for a year because he insisted on making a story based game (like all Bungie games) into an "open world" since it was the most popular and profitable buzz word at the time.