AOC E1659FWU 16" Portable LED Monitor

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?

Nope, sorry.