Let’s start with the basics, you could use LiveView without PubSub. Phoenix has a dependency in the underlying channel in order to provide some convenience, but that’s not core to how LiveView/Channels work. Digging in:

On the surface LiveView lets you write (mostly) Elixir to build rich UIs. The messaging that happens back/forth to the frontend/backend is not tied to PubSub. For example, clicking a button will send a message from client->server but PubSub is not involved.

But LiveView isn’t just a library to let you build rich UIs in Elixir. It has a real-time component that lets you do things like update the UI based on a change that happened somewhere else. This is “magical” in some ways, but is critically backed by PubSub.

Let’s say that user A is connected to server 1. An event happens over a webhook and arrives on server 2. User A is not connected to server 2, so it wouldn’t get the real-time update and your UI is stale (not real-time).

This is where PubSub comes in. When the event happens on server 2, you can broadcast that event over PubSub. Other servers receive that event and they see that there’s a listener that cares about the event (your LiveView process). The event is processed and the UI is updated—your app is real-time.

Just to be clear though, that’s optional. If you just want the rich UI building and worry about real-time later, then you likely won’t even think about PubSub other than the setup instructions.

edit1: Added bit about how Phoenix has a dependency on PubSub, so it’s required to use LiveView. Added bit about how you don’t need to worry about PubSub unless you want real-time updates

May I suggest you to read a book? Asking on a forum works best if you already have the right context and only need a nudge in the right direction.

The author of real-time Phoenix, @sb8244 has answered your question. So maybe you can start with that book.