Calling Diwali the Hindu festival of lights is true and flattens a great deal.
It runs across five days with distinct observances, Dhanteras, then Naraka Chaturdashi, then the main night, then a day variously kept as Govardhan Puja or the Gujarati new year, then Bhai Dooj for brothers and sisters, and which of these are emphasised varies sharply by region.
The story attached also varies. North India predominantly connects it to the return of Rama to Ayodhya after fourteen years of exile, with lamps lit to welcome him. In Bengal the night is given to Kali. In Gujarat and much of the mercantile west it is primarily the worship of Lakshmi and the opening of new account books, and the business year begins with it. In the south the emphasis falls on Krishna's defeat of Narakasura. These are not versions of one narrative; they are different festivals sharing a date and a name.
It is also not only Hindu. Jains keep the same night as the anniversary of Mahavira's attainment of liberation. Sikhs keep Bandi Chhor Divas, commemorating the release of Guru Hargobind from imprisonment at Gwalior along with fifty-two princes, and the Golden Temple is illuminated. Some Newar Buddhists in Nepal observe it. These communities are not celebrating a Hindu festival by courtesy; each has its own reason for the date.
The firework tradition is recent enough to be documented and is now a serious urban air quality problem, addressed by Indian court orders restricting the type and hours of use.