Please do yourself a favor, and download an install guide for Lubuntu/Ubuntu. It should walk you through configuring the system, including sound.
Also, please remember to update your system. It probably prompt you for an update. Do it. Bring up a terminal and type the following:
sudo apt update
sudo apt upgrade
That will probably prompt you to upgrade to the current 20.04 version. Not a bad idea to do so. Be patient, it replaces most of the software on your machine.
There is also a graphical Updater app that does this.
More advanced topic...Getting bit-perfect audio output means bypassing Pulse Audio and running ALSA instead. The ALSA system is used by Pulse Audio for drivers for soundcards and USB DACs, and should detect what you have connected. But the PulseAudio system that integrated desktop sound will tend to convert datastreams to 48kHz, so it either needs to be removed, or you need to configure your card in ALSA. This is probably the biggest pain. Dedicated Linux-based audio server software (like Daphile), already have this straightened out. I would initially get your USB DAC detected under PulseAudio so you know it works, and then worry about bit-perfect later.
As far as multiple desktops, that is the environment, allowing you to have different programs displayed on their own screens back before it became possible to hook up multiple monitors and working across screens. You can work on one desktop if you are more comfortable with that and just ignore the others.
Also, I don't know if you mean different desktop environments, i.e. the lightweight desktop on Lubuntu, or the stock Ubuntu. Often you are given a choice on the graphic login screen to select windows managers. Pick what Lubuntu uses (which should be the default).
As far as virtual machines, you need to choose, download, and install a virtual machine manager software package. There are several out there that can be downloaded. I recommend Virtualbox, which is openly developed and maintained, and there is good documentation for installing and using the software.
Oracle VM Virtualbox
https://www.virtualbox.org/This will require you to read through the documentation, there are quick install guides and FAQs for most of this information. It really cannot be spoon fed easily. Gotta do your research.
David