I believe each service has it's own queue, so producing an event would "fan-out" and add the event to each queue. That was my interpretation of why they were us

I believe each service has it's own queue, so producing an event would "fan-out" and add the event to each queue. That was my interpretation of why they were using both SNS and SQS - but I could be mistaken. If a particular service is not interested in a given event, it can just pluck it out of the queue and move onto processing the next message.