Amazon MSK and QLDB example - how is the response handled?

So considering this post https://aws.amazon.com/blogs/industries/building-a-core-banking-system-with-amazon-quantum-ledger-database/

• We are processing a realtime transaction (means that user swiped her card and is waiting on the "other" end for confirmation)
• We put the data to Amazon MSK which basically puts the flow into async mode
• Some microservice in Amazon EKS performs some actions and needs to "get back" to the waiting customer with approved/declined

How is that accomplished? Because if I understand correctly at some part (Transit Gateway?) the response "hangs" and the customer "waits". How does it get "notified" about the outcome?