Using VirtualBox to Build FreeBSD VM on Debian

Many virtualization software (aka hypervisors) can run on the host operating system of Linux. Notable open-source software includes VirtualBox, KVM, and Xen. This post outlines how to 1) install VirtualBox on Debian 9 (Stretch), and 2) build a FreeBSD virtual machine.
About VirtualBox
In 2010, Oracle acquired Sun Microsystems and, as a result, took over the ownership of several open-source technologies including VirtualBox and MySQL. Subsequently, VirtualBox was rebranded as Oracle VM VirtualBox, but the software has remained free and open-source.