The Overview with Fast Scan panel is part of the onboarding workflow introduced in 2.3.1. It will only be populated when this onboarding workflow is enabled and used - see Administration Center - Settings - Application Onboarding. See also Workflow - Application onboarding with Fast Scan.
The Architecture Preview section is a graphical representation of the delivered source code before an analysis is run - this is determined during the fast scan process. The section's primary aim is to help check the completeness of the source code that has been delivered: