The client and editor both have autoupdaters for assets.
Artists and musicians would be using other pieces of software for their work, but when they have something usable for the other people working on the project the assets they made can be added to the autoupdater.
Maps, quests, events, etcetera are all done via the editor, which will be connected to a server. Multiple people can be working on it different pieces of the game content at once (e.g. different maps, different NPCs).