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
VLC example:
flatpak remote-ls flathub | grep VLC flatpak install flathub org.videolan.VLC