Is the source code for the launcher out on github, becasue im altille confused regarding the launcher. Does it use the minecraft launcher to launch the modpacks, or does it has a custom launch proceed?
Pretty much. The FTB launcher just sets up a version (instance) of Minecraft with the mods and then sets the minecraft launcher to properly launch minecraft with the mods. It basically does what people can (and used to have to) do manually.