(VirtualBox simulates inserting a CD into the simulated optical drive of the VM, and Ubuntu auto-mounts that CD under "/media/a/").At the top of the "testUbuntu20 - Oracle VM VirtualBox" window, choose Devices > "Insert Guest Additions CD image." Other versions and other hosts should work similarly.) (This was tested on Ubuntu Desktop 20.04 LTS as a guest in VirtualBox-6.1 on a Windows 10 host.
This chapter assumes you already have Ubuntu installed in a VirtualBox virtual machine, perhaps using the process described in VirtualBox/Setting up a Virtual Machine/Ubuntu. Here we install the Guest Additions on an Ubuntu guest system running inside Virtualbox.