The United Nations uses three definitions for what constitutes a city, as not all cities in all jurisdictions are classified using the same criteria. Cities may be defined as the cities proper, their metropolitan regions, or the extent of their urban area. A complicating factor is that many large cities in the world have not only homeless or the unhoused, but also vast slum communities. This leads to official census data being less accurate in representing the actual number of residents in a given area.
In cities of the developed world urbanization traditionally exhibited a concentration of human activities and settlements around the downtown area, the so-called in-migration.throughout history. Most densely populated cities. Most populous national capitals.

Furthermore, visual representations like the one above help us fully grasp the concept of Most Populous City In The World.
This is Shanghai: What It's Like to Live in the World's Most Populous City is a 2015 non-fiction book by British author and illustrator Alexander Barrett, offering a firsthand account of expatriate life in Shanghai, China. Published by Microcosm Pub

This particular example perfectly highlights why Most Populous City In The World is so captivating.
Japan's second-largest city and tenth-most populous in the world is Osaka. Found in the south of Japan, on the island of Honshu, Osaka is renowned for its beauty and peaceful nature. Similar to Tokyo, Osaka is astonishingly quiet and clean despite its size.