There are no custom topics for Extension PubSub, if you want your backend to send data to your frontend you would use the Send Extension PubSub Message endpoint.
This allows your Extension to send up to 100 messages per minute per channel, each of up to 5KB, and either broadcasted to all users on the channel or whispered to specific users. Within the frontend you would use the Listen Helper function so that the client could listen for those messages.
The content of that message can be whatever you like, so in your case you would likely want to send the data to keep your Extension overlay updated, and maybe some metadata such as type of event so the client know what type of data it’s receiving and what to do with it.