A lot of SPAs do some level of client-side routing, so that if you open up the SPA and click on a link, rather than directing you to a different page, the app i

A lot of SPAs do some level of client-side routing, so that if you open up the SPA and click on a link, rather than directing you to a different page, the app instead updates the URL with Javascript and renders the content of the new page. There are various benefits and drawbacks to this approach, but it's quite common, and you can find tools like React Router that are designed to support this client-side routing. The problem is that when you only serve your app from `/`, and then the user navigates via the app to `/user/123`, the url gets updated, but if the user tries to reload the page, they'll end up on a 404 page because you're not serving the app from `/user/123`. There's a few ways to solve this, each again with tradeoffs, but one way is to have the server return 200 and the contents of the main page any time that the user requests an unknown route, under the assumption that the user is probably trying to navigate to a dynamic route handled by the frontend.