how to install ubuntu on a virtual machine code example

Example: how to install virtualbox in ubuntu

wget –q https://www.virtualbox.org/download/oracle_vbox_2016.asc -O- | sudo apt-key add -
#use codename of your linux version,for my case focal is codename for 
#ubuntu 20.04.1
sudo add-apt-repository "deb http://download.virtualbox.org/virtualbox/debian focal contrib"
#you can check codename of your system using command below
lsb_release -cs
sudo apt update && sudo apt install virtualbox-6.1