Short answers, pulled from the story.
The word Hindu first appeared in the 5th-century BCE DNa inscription of Darius I as a Persian geographical term for people living beyond the Indus River. This ancient record referred to the region known as Hapta Hindu, which corresponds to the Sanskrit Sapta Sindhu or land of seven rivers mentioned in the Rig Veda.
European merchants and colonists began referring to followers of Indian religions collectively as Hindus during the 18th century. British colonial laws and Orientalist scholarship codified diverse Indian traditions into a unified category called Hinduism during the 18th and 19th centuries.
There are approximately 1.17 billion Hindus worldwide representing 14.9% of global population with about 95% concentrated in India alone. Pew Research projects there will be 1.4 billion Hindus by 2050.
Varanasi serves as a documented sacred pilgrimage site within the Varanasimahatmya text embedded inside the Skanda Purana, with oldest versions dating to the 6th to 8th-century CE. Non-Hindu texts including memoirs of Chinese Buddhist and Persian Muslim travelers attest to the existence and significance of pilgrimages among Hindus by the later 1st millennium CE.
Modern Hindu nationalism was born in Maharashtra during the 1920s as a reaction to the Islamic Khilafat Movement wherein Indian Muslims championed the Turkish Ottoman sultan as Caliph after World War I. The movement developed concept of Hindutva in second half of 20th century after separation of India and Pakistan in 1947.