Is it were possible to have default routes in Next.js so all routes or a whitelist of routes will all go to a specific page.
For my project the idea is to use Next.js to build the marketing website, sign up and sign in process, and then to have a Next.js page that would boot up an SPA (probably using create-react-app and react-router) so that any route which isn't a Next.js page would assume that instead its a route in the SPA and would therefore direct to that page which would boot up the SPA
One way that seems to work is to create a custom 404 page. https://nextjs.org/docs/advanced-features/custom-error-page
Every route that is not a recognised next.js route will call this page, but will not redirect (change the browser URL) which is exactly what's wanted when booting up an SPA.