Discover is an software center that let's you manage software from multiple sources, including your operating system's software repository, Flatpak repos, the Snap store, or even AppImages from store.kde.org. Also, Discover allows you to find, install, and manage add-ons for Plasma and all your favorite KDE apps.
KDE Plasma Discover App Store
Flathub is an app store and build service for hundreds of apps which can be easily installed on any Linux distribution. Browse the apps online, from your app center or the command line.
Flathub App Store
Note: Flathub app search will be integrated in Discover, if you want to limit the app search to Flathub you can mark Flatpak as default by clicking on the star.
Flatseal is a graphical utility to review and modify permissions from your Flatpak applications. Get it on Flathub store.