How do they pass messages between them? what are the classes involved?
Do you understand what Smack and Openfire are?
Openfire is a XMPP based messaging server.
Smack is a XMPP client library.
XMPP is an XML based messaging standard so they pass XML back and forth.
You use Smack to send message to/from an Openfire server just like you would to any XMPP based server such as the servers run by Jabber.com and Google.com.
So the question you’re asking is basically how do I use Smack which is answered in the Smack documentation.