I'm creating a live-USB and don't understand precisely: what happens when I copy a file (or even a file system) directly to a device node (as opposed to a file system)?
The shell will open the device
The end result is that
In effect, the device node makes it appear that the low-level contents of your storage medium behave as a single file. You're writing into this "file", and therefore writing into the storage medium. This philosophy is known as "everything is a file" and is considered one of the defining features of a Unix system.