It looks like you are running XFCE instead of GNOME (the normal Ubuntu desktop). I’m not sure how that happened… but you an always just install another desktop.
For instance, you can try to make sure you have the ubuntu-desktop
or ubuntu-desktop-minimal
metapackage installed:
sudo apt install ubuntu-desktop-minimal
After that, the login manager should allow you to select the Ubuntu session rather than the XFCE one.
From the Discourse Blog:
The Linux desktop provides XDG Desktop Portals as a standardised way for applications to access resources that are outside of the sandbox. Applications that have been updated to use XDG Desktop Portals will continue to use them. Prompting is not intended to replace XDG Desktop Portals but to complement them by providing the desktop an alternative way to ask the user for permission. Either when an application has not been updated to use XDG Desktop Portals, or when it makes access requests not covered by XDG Desktop Portals.
Since prompting works at the syscall level, it does not require an application’s awareness or cooperation to work and extends the set of applications that can be run inside of a sandbox, allowing for a safer desktop. It is designed to enable desktop applications to take full advantage of snap packaging that might otherwise require classic confinement.
So this looks like it complements and not replaces the XDG Desktop Portals, especially for applications that have not implemented the Portals. It allows you to still run those applications in confinement while providing some more granular access controls.
This is a great summary. Thanks!