All of that is configured into the rclone config in advance. Some notes on the setup for Duplicati:
Rclone backend documentation
Rclone local repository
and if you want code:
https://github.com/duplicati/duplicati/tree/master/Duplicati/Library/Backend/Rclone
Note that it implements IBackend not IStreamingBackend, so UI might be a little bit less featured.
If you feel current Duplicati manual could use clarification, pull requests can be submitted to it.
Actually, pull requests can be submitted to Duplicati too, and volunteers are very much sought.