They're generated client-side using KaTeX[1]. That said the entire page is generated using JS meaning that someone visiting the site with JS disabled or using a text web browser will be greeted with a blank page. Nevertheless no-JS versions seem to be shown to bots since Google caches a plain HTML version of the same page[2].
Maybe it's just an oversight then when inline and block math was rolled out. KaTeX has a `renderToString` function that can be used to server-side render the LaTeX.
On an other hand, Notion doesn't seem built to serve read-only webpages like a static blog or Medium.com: the expectation is that you'll use the editor, so assumption is javascript is enabled and the editor itself can be used to render a read-only view from JSON or however they're keeping document state.
Opening dev tools on the website, it looks like they're just using Webpack (like React CRA?); not sure if they're changing the javascript bundle per page like with Next.js.
It would make sense to not have server-side rendering if you're building both browser and desktop apps, since that would mean avoiding a separate framework only for the browser.