The whole fake neighborhood was built atop a Boeing factory south of downtown Seattle, and was the cumulative size of eight football fields. Houses, streets, and plants were assembled out of ply wood, cardboard, etc. to act as props and give the town the most convincing appearance possible.
The thought was that if any Japanese bombers made it this far east, they would confuse the plant for a quiet residential neighborhood, and as a result, not bomb it.
The workers at the plant had to use secret underground tunnels to reach nearby and also disguised cafeterias, classrooms, etc. Just another example of how war changes the landscape of a country.