Short answers, pulled from the story.
The Indo-Gangetic Plain is a fertile alluvial plain spanning 700,000 square kilometres across the northern and north-eastern Indian subcontinent. It encompasses northern and eastern India, eastern Pakistan, southern Nepal, and almost all of Bangladesh, stretching from the Himalayas in the north to the Vindhyas in the south and from North East India westward to the Iranian border.
The plain is fertile because it consists of the world's largest expanse of uninterrupted alluvium, deposited over millennia by the Indus, Ganges, and Brahmaputra rivers. Its rich water resources and alluvial soils make it one of the world's most intensely farmed areas.
The Indus Valley Civilisation established settlements in the region as early as 3000 BCE. Later empires that had their political and demographic centres here include the Maurya Empire (4th to 2nd century BCE), the Kushan Empire (middle 1st century CE), the Gupta Empire (4th to 7th century CE), the Delhi Sultanate (13th century CE), and the Mughal Empire, founded by Babur in 1526 CE.
The Ganges-Brahmaputra Delta is the world's largest river delta, formed where the Ganges and Brahmaputra enter the Bay of Bengal. It covers more than 100,000 square kilometres, with about two-thirds in Bangladesh, and is home to more than 130 million people.
The plain is divided into four strips running south from the Himalayas: the Bhabar, a narrow porous band of rocks where rivers disappear underground; the Terai, a wetter zone of new silt and evergreen vegetation; the Bangar, the main fertile belt of older alluvium; and the Khadir, low-lying areas of new silt at the southern edge.
Competing theories proposed the depression was a furrow predating both the Himalayas and the Deccan Plateau (Darashaw Wadia), a large syncline formed by the Himalayan advance (Edward Suess), or a deep rift in the Earth's crust (Sydney Burrard). More recent research holds that sediment from the prehistoric Tethys Sea folded northward with the drift of the Indian plate, and the trough deepened when the Himalayas emerged.