In this video, I will show you an actual install of Linux Ubuntu 20.04 to an external (SSD, HDD, USB)
link for how to create a Bootable USB drive:https://www.youtube.com/watch?v=JkCwuMXQq9s&t=51s
link for How to Boot From a USB Device:
https://www.lifewire.com/how-to-boot-from-a-usb-device-2626091