Does this sync data with the roster and update addons when it is started up, or is that a completely manual process?

If it is always manual, can it be set up so that it always syncs all required addons and uploads the character data on launch?

By default uploads happen as soon as the file changes (MD5 hash of the file is monitored every X seconds - where X can be configured, by default 20 seconds).

Updating addons is a manual process (well, it checks which addons are updatable automatically, but it needs to be started manually). The reason for this is that checking a lot of addons can be very CPU and IO intensive. It would severely affect your gameplay if it were automated.