For sandboxing user content they need to be on their own origin, otherwise by opening the wrong notebook your account could be taken over. Ideally you also have a unique origin per user as well, as otherwise they would share LocalStorage and cookies. So here every user gets <username>.notebook.host for their notebooks (in the future I might completely randomly generate it instead). This is also the domain used for static embedding.

starboard.gg is used as the main entrypoint: it is just a static website (SPA-like). And the APIs are hosted on starboardapi.com. Those two could technically be combined.

And finally there's another one.. starboardproxy.com which acts as a CORS proxy for use in notebooks.