What is the difference between the official (generic/vanilla) Ubuntu and the one which ships with the Dell Precision 5520?
Does the latter contain any non-free software, for example?
It displays Dell's License Agreement at the start:
dell iso image contains dell-recovery package plus under /installs folder contains the drivers for your laptop, they also contain or set up /etc/apt/sources.d/ with dell apt repo to update packages.
last change is that the executable they run in the ram image is installation script and do not alive "live image" mode where you can play around with it, most chances because they do some setups, the installation is "hands free" but what's not is the regular ubuntu installation and what is is dell setting up drivers and defs for u like the nvidia-intel combo video card driver.
if u have the .iso image, the original one for precision 5520 i'de love to have that.