Thx for reply. Now I see more clear all the system.
The desktop app is used by the streamer, the viewer will use the extension to interact with it.
The scenario is:
Streamer PC with my desktop App ← WebSocket (Twitch or my own systems) → Extension on Twitch streamer page
I know that the best environment is using my own servers, but I was wondering if I can use what twitch offers me since it is better distributed than what I could do by my self (with low-to-zero budget).
But I see that I need to use Extension Secret to access pubsub, so I definitely need to develop my own system and can’t put that secret on desktop app 
All this comes from a simple wondering. I can use a server to do server-to-server communication, but that would be my single point of failure. So I need to add more server to distribute the loads and the price to get all this machines goes higher