How did people belonging to different communities, regions or language groups develop a sense of collective belonging?

There are multiple factors which collectively brings the entire nation together despite huge diversities and making it a one nation state. Almost every nation in the world has gone through the same process. If we talk of Indian context, the factors responsible for it are mentioned below:-

(i) Struggling for the same issue:- The most important factor responsible for arousing the sense of nationalism among the Indians was their united struggle against the Britishers.
(ii) Cultural processes:- There were also a variety of cultural processes through which nationalism came into people’s imagination. Indian history and fiction, folklore (customs or beliefs) and songs, popular prints and symbols, etc. played an important part in arousing the feeling of nationalism among Indians.
(ιιι)ΒηαρατΜατα:- The identity of India came into existence with the visual associated with the image of Bharat Mata, which was created in 1870 by Bankim Chandra Chattopadhyay, who wrote ‘Vande Mataram' as a hymn to the Motherland. Afterward, inspired by the Swadeshi Movement, Rabindranath Tagore painted his famous image of Bharat Mata.
(iv) The revival of Indian Folklore:- The idea of nationalism was also developed by revitalizing the Indian Folklore.
In late-nineteenth-century India, nationalists began recording folk tales sung by bards (poets), and they toured village to village to gather folk songs and legends. This was done in order to promote the traditional culture of India that was corrupted and damaged by western forces. It was essential to preserve this folk tradition in order to discover one’s national identity and restore a sense of pride in one’s past.
(v) Reinterpretation of History:- By the end of the 19th century, many Indians started feeling that to evoke a sense of pride in the nation, Indian history had to be thought about differently. The British saw Indians as backward, primitive and incapable of governing themselves. So, Indians began looking into the past to discover India’s great achievements. They wrote about the glorious achievements and developments in ancient times. The nationalist historians urged the readers to be proud of India’s great achievements in the past and its struggle to change the miserable conditions of life under British rule.