I got a pretty general sys design question and was asked to dive into the specifics of a part of implementing a distributed data store when I mentioned that I would need one. this was for SDE II. Got asked on my phone screen and the onsite, both by the same person (hiring manager)
Had a good friend that really described his Amazon interview process. He did say that a LOT of it was how you thought and the approach to problems (even problems that were non-technical in his SDE interview)
System Design sessions are often team specific, and probably specific to the current team's projects. Do your research on what team builds/does before the interview, it might help. There are different teams at Amazon that range from building databases to building simple MVC applications - they will have different engineering needs to fill.
This is true but it's unlikely you will need any domain knowledge to solve the problem. Most important this is make no assumptions about anything and ask a clarifying questions about anything. If you are good about teasing out the requirements the design itself is usually straightforward