Short answers, pulled from the story.
The Indo-Gangetic Plain formed as a depression between the Himalayas and the Deccan Plateau filled with alluvium. Recent research indicates sediment from the pre-historic Tethys Sea folded northward due to plate drift while a trough emerged when the Himalayas rose in the north.
Human settlement flourished here starting around 3000 BCE with the Indus Valley Civilisation. During the Vedic period between 1500 and 600 BCE, the area was known as Aryavarta or Land of the Aryans.
The plains encompass northern and eastern India, eastern Pakistan, southern Nepal, and almost all of Bangladesh. They cover states like Rajasthan, Punjab, Haryana, Uttar Pradesh, Bihar, West Bengal, and Assam within the Indian subcontinent.
These four zones include Bhabar which is porous below the foothills, Terai where streams reappear, Bangar consisting of older alluvium, and Khadir composed of new silt. These regions define the physical character of the entire plain from west to east.
Nearly one-seventh of the global population lives within this fertile plain. More than 130 million inhabitants reside in the Ganges-Brahmaputra Delta alone making it one of the most densely populated regions in the entire world.