metropolis vs Motor City

metropolis and Motor City both are nouns.

Word	Noun	Adjective	Verb	Adverb
metropolis	Yes	No	No	No
Motor City	Yes	No	No	No

metropolis (noun)	Motor City (noun)
people living in a large densely populated municipality	the largest city in Michigan and a major Great Lakes port; center of the United States automobile industry; located in southeastern Michigan on the Detroit river across from Windsor
a large and densely populated urban area; may include several independent administrative districts