current Debian system must have 'virtual box' from which another Linux distro will be able to start

How can the current running Debian (MX Linux) system have a 'virtual box' from which another Linux distro will be able to start/boot/launch as it's supposedly booted/started from a USB disk ?

It arises from the difficulty to install Funtoo (a better Gentoo OS) while a HTML viewer requring GUI capability, must be extant at the same time to read the Funtoo installation documentation
https://www.funtoo.org/Install