What does a City planner do?

A city planner designs the city for instance they would decide the layout of the city. In other Words they decide where the buildings are located. If there is a lake and the city planner would often start there and work their way inland. In some cases the cities are modeled after other cities that have already been created.Have be told that they can make up what like the the flag is and what they mean. Not sure how factual that is but is probable.