A channel == a process

If you broadcast to a topic unique to a given user, say "user:123", only that user will receive the message. If you’re asking how to message to a single user 123’s devices, then you can use Phoenix Presence, but we need to know more info to know what your exact usecase is. Generally it’s just fine to broadcast to a private user topic since you usually want all tabs/devices the user has joined to receive the message.

Nope, a process is spawned for each topic for each user. You can join more topics from inside a topic though, and ‘those’ will not create more processes, but anytime the javascript sides connects to a topic for every single user then a new process is created for that specific topic/connection by default.

It’s at: Phoenix.Endpoint — Phoenix v1.8.8

To join you’d actually do:

def join("users", %{"id" => id}, socket) do
  MyEndpoint.subscribe("users:" <> id)
end

At which point this same "users" process will get messages for "users:"<>id, but it will not automatically forward them to the client, to do that you just listen for them (just like any other PubSub message, Phoenix Channels are just Phoenix.PubSub underneath) and then push them to the client however you want.