I've been working on portals recently and one of the issues for me was that the documentation just didn't quite hit the sweet spot. At least the bits I found were either too high-level or too implementation-specific. So here's a set of notes on how a portal works, in the hope that this is actually correct.
First, Portals are supposed to be a way for sandboxed applications (flatpaks) to trigger functionality they don't have direct access too. The prime example: opening a file without the application having access to $HOME. This is done by the applications talking to portals instead of doing the functionality themselves.
There is really only one portal process: /usr/libexec/xdg-desktop-portal, started as a systemd user service. That process owns a DBus bus name (org.freedesktop.portal.Desktop) and an object on that name (/org/freedesktop/portal/desktop). You can see that bus name and object with D-Feet, from DBus' POV there's nothing special about it. What makes it the portal is simply that the application running inside the sandbox can talk to that DBus name and thus call the various methods. Obviously the xdg-desktop-portal needs to run outside the sandbox to do its things.
There are multiple portal interfaces, all available on that one object. Those interfaces have names like org.freedesktop.portal.FileChooser (to open/save files). The xdg-desktop-portal implements those interfaces and thus handles any method calls on those interfaces. So where an application is sandboxed, it doesn't implement the functionality itself, it instead calls e.g. the OpenFile() method on the org.freedesktop.portal.FileChooser interface. Then it gets an fd back and can read the content of that file without needing full access to the file system.
Some interfaces are fully handled within xdg-desktop-portal. For example, the Camera portal checks a few things internally, pops up a dialog for the user to confirm access to if needed  but otherwise there's nothing else involved with this specific method call.
Other interfaces have a backend "implementation" DBus interface. For example, the org.freedesktop.portal.FileChooser interface has a org.freedesktop.impl.portal.FileChooser (notice the "impl") counterpart. xdg-desktop-portal does not implement those impl.portals. xdg-desktop-portal instead routes the DBus calls to the respective "impl.portal". Your sandboxed application calls OpenFile(), xdg-desktop-portal now calls OpenFile() on org.freedesktop.impl.portal.FileChooser. That interface returns a value, xdg-desktop-portal extracts it and returns it back to the application in respones to the original OpenFile() call.
What provides those impl.portals doesn't matter to xdg-desktop-portal, and this is where things are hot-swappable.
GTK and Qt both provide (some of) those impl portals, There are GTK and Qt-specific portals with xdg-desktop-portal-gtk and xdg-desktop-portal-kde but another one is provided by GNOME Shell directly. You can check the files in /usr/share/xdg-desktop-portal/portals/ and see which impl portal is provided on which bus name. The reason those impl.portals exist is so they can be native to the desktop environment - regardless what application you're running and with a generic xdg-desktop-portal, you see the native file chooser dialog for your desktop environment.
So the full call sequence is:
- At startup, xdg-desktop-portal parses the /usr/libexec/xdg-desktop-portal/*.portal files to know which impl.portal interface is provided on which bus name
- The application calls OpenFile() on the org.freedesktop.portal.FileChooser interface on the object path /org/freedesktop/portal/desktop. It can do so because the bus name this object sits on is not restricted by the sandbox
- xdg-desktop-portal receives that call. This is portal with an impl.portal so xdg-desktop-portal calls OpenFile() on the bus name that provides the org.freedesktop.impl.portal.FileChooser interface (as previously established by reading the *.portal files)
- Assuming xdg-desktop-portal-gtk provides that portal at the moment, that process now pops up a GTK FileChooser dialog that runs outside the sandbox. User selects a file
- xdg-desktop-portal-gtk sends back the fd for the file to the xdg-desktop-portal, and the impl.portal parts are done
- xdg-desktop-portal receives that fd and sends it back as reply to the OpenFile() method in the normal portal
- The application receives the fd and can read the file now
Finally: because of DBus restrictions, the various methods in the portal interfaces don't just reply with values. Instead, the xdg-desktop-portal creates a new org.freedesktop.portal.Request object and returns the object path for that. Once that's done the method is complete from DBus' POV. When the actual return value arrives (e.g. the fd), that value is passed via a signal on that Request object, which is then destroyed. This roundabout way is done for purely technical reasons, regular DBus methods would time out while the user picks a file path.
Anyway. Maybe this helps someone understanding how the portal bits fit together.
 it does so using another portal but let's ignore that
 not really hot-swappable though. You need to restart xdg-desktop-portal but not your host. So luke-warm-swappable only
Edit Sep 01: clarify that it's not GTK/Qt providing the portals, but xdg-desktop-portal-gtk and -kde