Hi @Lachie,
ArduSub is the flight controller/autopilot (e.g. Pixhawk) firmware. The software that runs on the Companion computer is just called the Companion software.
If you’re interested, here’s an overview of the software components 
The current stable Companion is unlikely to be pleasant to try to convert to a different board than the intended RPi3B. Your best bet is trying the new Companion Beta, which is made to be much more self-contained and platform independent.
That said, it’s still been developed on Raspberry Pi, so isn’t yet at the stage where using it on other platforms is simple. Probably the best starting point is the files in the install directory, and in particular the install.sh installer script, which handles installation on different Raspberry Pi models. That should hopefully cover most of the required setup steps, some of which you’ll be able to replicate directly on a Jetson Nano, but others will likely require finding something equivalent.
From what I understand we’re hoping to properly support other board types down the line, but at this stage it’s still in beta on our recommended hardware, so understandably testing that and developing it to stable takes priority over supporting other boards.