How/Where does KDE Discover load appstream metadata from?

Looking through the source, I understand that KDE pulls a list of the “Featured” apps from here, but that only provides the app package names. From where does Discover get the actual info and metadata like summaries and links to demo images for each of these apps?

That information is locally installed on your system, by your distro. It usually lives at /usr/share/metainfo/.