Flatpak is a universal Linux package management system, that is more agnostic than the Linux distributions package management systems. Flatpak currently only works with a desk top environment, it is not for servers.
There are other universal package management systems available. One popular example is snap, however this is from Ubuntu and seems propriety in nature. (This was another reason why I moved away from Ubuntu as the snap packaging system seems to almost be forced on recent Ubuntu distributions, as well as other bloat wear. My perception anyway, Debian is certainly less bloated..)

The following are some basic flatpak commands:

• flatpak update to update the flatpak
• flatpak list to list install flatpaks
• flatpak search package to search the flatpak repository for for app named package
• flatpak uninstall --unused to remove unused flatpaks
• flatpak repair to repair flatpak system

Notes:

1. Flatpak seems to operate without sudo. Not sure if I had to setup so, or was just like that.
2. There are other method to install flatpaks, for diferent repositories, downloaded files and via web pages, I have not investigated this, to date flatpakhub seems to have had all the apps I needed.