Short answers, pulled from the story.
Mexico City is the capital and most populous city of Mexico, and the most populous city in North America. According to the 2020 census the city proper had 9,209,944 people, while Greater Mexico City reached 21,804,515, the world's 15th-largest metropolitan area.
Mexico City began as Tenochtitlan, founded by the Mexica people on islands in Lake Texcoco around 1325 or 1327. It is the oldest capital city in the Americas and one of only two founded by Indigenous peoples.
Mexico City rests on the heavily saturated clay of the former Lake Texcoco, and over-extraction of groundwater collapses that soft base. The city has sunk as much as 9 meters in some areas and continues to sink almost 50 centimeters every year.
By the 1990s Mexico City was infamous as one of the world's most polluted cities, but it has since become a model for lowering pollution. By 2014 carbon monoxide had dropped sharply and sulfur dioxide and nitrogen dioxide were about a third of their 1992 levels, helped by emissions inspections, fuel reformulation, and programs like Hoy No Circula.
On the 29th of January 2016 the Federal District was officially renamed Ciudad de México, or CDMX. On that date the city began transitioning toward becoming the country's 32nd federal entity, with autonomy comparable to a state.
Chilango is the informal name for a Mexico City resident. It was historically used as an insult to suggest a loud, arrogant, ill-mannered person, but many residents now proudly embrace it. The more polite formal term is capitalino.
Hernán Cortés reached Tenochtitlan on the 8th of November 1519 and was greeted by Moctezuma II. After the uprising of La Noche Triste on the 30th of June 1520, Cortés laid a three-month siege beginning in May 1521. Cuauhtémoc surrendered in August 1521 and the Spaniards practically razed the city before rebuilding it.