You will probably be successful with calling PubSub.subscribe inside of your Transport process. This will receive a handle_info callback whenever an event is received over PubSub. You must subscribe to specific topics, which the book goes into detail about.
Think of PubSub as a layer that Phoenix provides. It sits below Channels, and Channels are built on top of it. If you don’t use Channels you can still benefit from PubSub because it’s a completely separate layer. You just need to call the right functions to use it.