I was asked a very well-known system design question which is frequently mentioned in other reviews. My advice is not to learn off one solution but rather look at all the main components of system design and understand each of them. It's very important to be able to talk about your considerations and possible trade-offs a little so don't just learn something and not be able to answer the 'why?' if asked. The interviewers may choose to drill into a particular topic a little more e.g. API design or hashing algorithm etc. Make sure you understand the requirements early and ask questions if needed to confirm details and assumptions. Thinking out loud really helps the interviewers understand your process and they can steer you in the right direction if needed - they are genuinely there to help you through the interview as well so work with them. Practising on a whiteboard beforehand really helps get familiar with the task.