Hub is a platform that orchestrate work of already installed vulnerability scanners within Your network. You are registering those scanners in one place and You can manage scans though this platform. In order to create and request a scan with given scope You need to call an API (with curl or provided applications)
Flow is platform that already has preinstalled set of most popular opensource scanners and itegration with them. In order to request for a scan You need to configure proper webhook on Your gitlab project.