The list below includes some of these folks I know personally, some just by reputation- but collectively they’ve grown products up to millions, 10s of millions, and in some cases, 100M+ users.
Typically they use quantitatively-oriented techniques centered on virality across different channels such as iOS, Facebook, email, etc. There’s lots of iteration, A/B testing, and experimentation involved. There’s also really great growth hackers centered around SEO, SEM/ad arb, and other techniques, but for the most part I’m just listing out the folks around quant-based virality. The important thing about virality is, it’s free So it’s an important skill for startups.