What is the difference between XMPP and FCM and Websocket?

Question

Android

I'm trying to understand how chats work in Android applications. I read it but didn't understand it.
What is the difference between XMPP(jabber) and FCM (Firbase CM) and Websocket/Socket.io. That is, I can’t figure out which layers should go through, for example, a message from a client application to another client. Which of the following technologies should be used? What should be the structure of all these technologies, what should follow what?

UPD: Are there any books, articles on this? By the way all these server things work, as I understand protocols, etc., how are all systems connected to each other? To more or less lay the foundations for it all. THX!

Answer 1

XMPP is an application layer protocol.
Websocket - transport layer.