This page provides an overview of the Communardo SharePoint Connector for Confluence Cloud and the components it interacts with.
Image 1: High Level Architecture for SharePoint Online (click to enlarge)
All communication between Confluence and SharePoint is done by the client of the end user (i.e. browser). Direct communication between these two ecosystems is not necessary.
All requests to the Microsoft Graph API are done in the context of the current user (see App Permissions (Cloud)).