A normal SMS goes from one phone to one phone. Bulk SMS goes from a business system to many phones at once. Instead of typing a message on a handset, the business logs in to an SMS panel or calls an SMS API, picks a list of numbers, and the SMS gateway hands every message to the right mobile operator (Jio, Airtel, Vi or BSNL) for delivery.
The message arrives in the normal SMS inbox with a short sender name, such as ABCBNK, instead of a ten-digit number. It works on every mobile phone, including basic feature phones, and does not need the internet or an app. That is why banks, schools, hospitals, retailers and government offices in India still rely on bulk SMS for alerts, OTPs and announcements.