They don't really need to do anything sneaky to figure out what to build next. They generally build things that their customers are banging down their doors asking them to build.

The main difference with AWS vs Azure or Google is that they have been focused on the space longer and they have really optimized the rate at which they can build out a new scalable, reliable, secure, mananged service.

Sure, the UI/UX is usually not great, and the documentation is imperfect and there are features missing, but they get the hard things right, they charge per second/hour/GB, and they improve over time.