If someone comes to you and says: "I want to build a platform that does WhizzyWhatsIt for my customers, it has to be on AWS so it's mingled with my existing infrastructure. It needs to provide an admin portal so that I can set WhizzyWhatsIt prices and watch user acquisition make my bank account go brrrrrtt. It needs the ability for my quazi-illegal affiliate marketing ring to be able to whitelabel and brand it as their own for a cut of the profits."

This is obviously satire but there's a clear ask, some features, from there you know what you need to have to even achieve those features, what project management process would you employ? Agile? Waterfall? Agile-fall? Kanban? Call me in 6 months?