Follow these simple steps to start using Flatpak
Flatpak support is built in by default from KDE neon 19 and later. To activate the Flathub repository in Discover, just follow the instructions below:
Open Discover and click on Settings (lower left corner).
Check that in the Flatpak section the box is checked.
Note: with this 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.