If an agent gets assigned to a case handled by a chatbot, there are two ways how to get the conversation back to the chatbot.
Deeplinks allow the agents to handover back to specific flows. Check out the guides in the link.
The Case is closed
The only other scenario where the Chatbot would engage with that customer is once the case is set as Closed and the customer writes back again, creating a new case that will trigger the Chatbot Welcome message.