can I use this as a second monitor connected to my laptop, where the first monitor is not a USB connected monitor?
That is how a lot of people use them - as a 3rd monitor.
Curious if this would function as an external display for Canon DSLR
Very unlikely as there isn’t a way to load the drivers onto your camera.
There are display drivers for these platforms: Windows, MacOS and Linux (Android, ChromeOS, Debian/Ubuntu). Anything else isn’t officially supported:
Can you use this with a Fire Stick?