Need advice for onsite interview at Meta

Good question, I was one of the people who helped get the product architecture interview going. At their core they are similar, they are both system design interviews. "Systems Design" is infra-focused, more back of the envelop math, more focused on performance and scaling. This is really for infrastructure, systems, production engineers. "Product Architecture" is like a full stack system design interviews. You need to be able to make a box diagram of the entire system, but a little more focus on the APIs between the boxes, and a little more focused on how "real world use cases" will impact the entire system end to end. For example, "design instagram". You probably start with a similar box diagram. The system one will go more into scaling photo storage, the feed infrastructure. The product one might go more in the Feed API and pagination. You might be asked to think of use cases that break the system and how to handle those at a high level strategy. The biggest difference is that the product one is typically with a product engineer and the system one is with a infra engineer.