Read about flatpak command reference here
See flathub site for bundled apps and tutorial how to add their repo.
Running flatpak commands as user (--user) may return errors.
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
Skype example:
flatpak remote-ls flathub | grep skype com.skype.Client com.skype.Client.Debug com.skype.Client.Sources flatpak install flathub com.skype.Client