To have a Virtualbox shared folder on the Ubuntu guest's desktop we will have to create a shared folder to then mount it. See these related questions:
If this was successful we need to give our Ubuntu user privileges to read (or write) these shared folders by granting them access to the group
vboxsf. Run this command from a terminal in the guest:
sudo usermod -aG vboxsf <username>
Both, shared folders auto-mounting to
/media/<username>/sf_<sharename>, and user membership in the group
vboxsf need a reboot of the guest.
We can then simply drag and drop the shared folder to the guest's desktop for easy access:
To access from guest applications we give the mountpoint (i.e. the path) to the share (when auto-mounted in
/media/<username>/sf_<sharename>) in the file-selector.