Smuxi consists of 3 main components: the engine, a frontend and the server.
The Smuxi engine includes all logic that is needed to initiate and maintain connections to chat services like IRC, Twitter, XMPP/Jabber and so on. All message handling, highlights and configuration also happens here. This is what makes Smuxi special compared to all other IRC or IM clients. The Smuxi engine is separate from the user interface layer. This allows Smuxi to host a engine remotely using the smuxi-server (aka remote engine) or to host it in the same process as the Smuxi frontend (aka local engine).
This is an application that knows how to connect to or spawn a Smuxi engine. A Smuxi remote engine can be used by many frontends at the same time. The idea is that you can switch between computers and keep using Smuxi fluently. The Smuxi API is used to achieve this.
So far two implementations of a Smuxi frontend exist: the GNOME frontend and the STFL frontend.
One could think, the smuxi-server must be some complex program, but it really is just a 10 lines of code wrapper to spawn a Smuxi engine and provide it to the network so frontends can connect to it.
Since the remote engine keeps running inside the smuxi-server while frontends can be closed, all chat connections, messages and other state stays alive. When a frontend connects to that smuxi-server it will sync/pull all that information and it seems like you never closed the frontend.
How a server is configured, started and stopped can be found in the server documentation.
Smuxi stores its settings in 2 INI files. INI files contain of sections and key/value pairs. All settings that are not related to state of the user interface are stored centrally in the Smuxi Engine. This means all frontends consume the same settings automatically. This allows the same user experience across all Smuxi connected devices.
The location of the INI files is $HOME/.config/smuxi
This file contains all settings of all Smuxi profiles.
This file contains the settings related to Smuxi remote entries and user interface state.