The actual content of your notes (we call them cards) is Markdown and is stored as such, but the source-of-truth for the notes themselves is our servers (not in the file-system), as our priority is providing a seamless experience on all your devices (or anywhere with a web connection), so the app was actually originally built to be "online-only". It's become much more offline-friendly over the years (your entire card collection actually is stored on your system for offline use / search is entirely offline / etc) but offline is still not 100% there yet, which is what the upcoming 3.0 release will enable.
Within the app we provide comprehensive export options[1] (you can export as markdown or as the actual JSON representation of a card that we use under-the-hood).
We also have a public API[2] that allows you to do whatever you want with your cards at any time with an API key. Currently the docs for this are still only an OpenAPI spec (and not an entirely complete one at that), but again that is something we are improving with the 3.0 release / subsequent releases.
So I wouldn't really describe ours as a closed system in the sense that Evernote was. In the case that Supernotes is ever acquired / shutdown / other black swan event for users, you should just be able to dump your cards into something like Obsidian and it will work pretty well, as we almost entirely respect the CommonMark spec.
Evernote being a cautionary tale, I think you understand why no one would or should move their notes to closed system.
Being based on Markdown means notes are stored somewhere in the file system, like Obsidian?