Dropbox is only available on Kobo Forma, Sage, and Elipsa. It isn't available in more popular Libra 2 or Aura HD.
It's not native, you're right. Many of us have been using Kobocloud for years to automatically sync to cloud drive services: https://github.com/fsantini/KoboCloud
I won't lie, I was excited to see the technical solution here - but this appears to be a stack of horrific shell scripts fixing everything via curl. Is it dependable, in your experience?