Short answers, pulled from the story.
Delhi, officially the National Capital Territory of Delhi, is a megacity and union territory of India that contains New Delhi, the capital of India. Delhi became a union territory on the 1st of November 1956 and the National Capital Territory in 1995, and its NCT covers an area of about 1,484 square kilometres.
According to the 2011 census, Delhi's city proper population was over 11 million, while the National Capital Territory's population was about 16.8 million. The wider urban agglomeration in the National Capital Region has an estimated population of over 28 million, making it the largest metropolitan area in India.
From the early 13th century until the mid-19th century, Delhi was the capital of the Delhi Sultanate and the Mughal Empire, which covered large parts of South Asia. The Delhi Sultanate was founded by Qutb-ud-din Aibak after 1206, and the Mughal Empire was founded by Babur in 1526.
In 1911 it was announced that the capital of the British Indian Empire would move from Calcutta to Delhi, formally transferred on the 12th of December 1911. The name New Delhi was given in 1927, and the new capital was inaugurated on the 13th of February 1931.
The World Health Organization named Delhi the most polluted city in the world in 2014, and a 2018 Ministry of Earth Sciences paper attributed almost 41% of its PM2.5 to vehicular emissions, 21.5% to dust and fire, and 18% to industries. Air quality crashes to Very Poor, Severe, or Hazardous levels between October and December due to stubble burning, Diwali firecrackers, and cold weather.
During the partition of 1947, around five hundred thousand Hindu and Sikh refugees mainly from West Punjab migrated to Delhi, while around three hundred thousand Muslim residents migrated to Pakistan. Delhi was transformed from a Mughal city to a Punjabi one, losing two-thirds of its Muslim population.
Delhi has three UNESCO World Heritage Sites, the Qutub Minar, Humayun's Tomb, and the Red Fort, all dating from the Delhi Sultanate and Mughal periods. Other landmarks include the Jama Masjid, India's largest mosque built in 1656, and the Lotus Temple, a Baha'i House of Worship completed in 1986.