Flatpak is a technology for packaging and distributing standalone dekstop
applications. It uses interfaces called "portals" to communicate with the
rest of the system.

xdg-desktop-portal works by exposing a series of D-Bus interfaces known as
portals under a well-known name (org.freedesktop.portal.Desktop) and object
path (/org/freedesktop/portal/desktop). The portal interfaces include APIs for
file access, opening URIs, printing and others.