Pages are created using templates from your nostr events (posts, articles, etc) fetched from whichever relays you wrote them to. Our engine allows to generate webpages on the server and in the browser - we pre-generate some pages for your site on our servers to make them visible to Google and to provide nice link previews - but after the first page is loaded into user's browser, our engine starts working inside the browser - fetching events directly from relays and turning them into webpages that are shown to users. It's sustainable because most of the work most of the time is done by the browsers talking directly to relays, similar to how other nostr clients work.