Please look over Duplicati components at the top of the Installation page for a summary of that.
Using Duplicati from the Command Line gets deeper into how one uses that to run backup, etc.
Using the Graphical User Interface talks more about server part, since that’s what Web UI uses.
Forum articles category has Different ways to make a Duplicati backup. Duplicati is very flexible.
EDIT:
… which can make it more confusing than something less so. If you have questions, please ask.