Kubuntu's software management suite is called Discover (or plasma-discover if you're using a terminal) and is analogous to the Ubuntu Software Center (or gnome-software). Its main panel displays a list of featured applications. How is this list generated?
Currently, in Kubuntu 18.04, the featured applications are:
[ "appstream://org.kde.krita.desktop", "appstream://org.kde.digikam.desktop", "appstream://org.kde.kdenlive.desktop", "appstream://org.kde.ktorrent.desktop", "appstream://org.kde.gcompris.desktop", "appstream://org.kde.kmymoney.desktop", "appstream://org.kde.kolourpaint.desktop", "appstream://blender.desktop", "appstream://inkscape.desktop", "appstream://org.kde.development" ]
Some of them can be seen in the image below:
The Reddit commenter also provided some background regarding how such a list is generated.
This is where AppStream comes in. See What is AppStream? Why is it installed? What is it doing?. The data collected by AppStream includes reviews and votes on those reviews: see this and this for more including what measures are taken to verify genuineness.