The short answer
Nithin Kamath has written it plainly: Zerodha does not advertise, does not have a sales team with targets, and does not do much marketing. What it did instead was price flat and low, which made the product itself the thing people told friends about, and roughly 30 percent of its users have come through referrals. It also built Varsity, a free, deep, plain language library of market lessons with no paywall and no ads, which brought in people long before they were ready to open an account. The Kite API turned developers into a distribution channel, since anyone building a trading tool on top needed a Zerodha account. And it stayed at about 1,000 people while the business grew five or six times, which is what let the economics work without spend.