The thing that makes the client-side invocation return the same data as the server-side one is that the results of calling fetch during SSR are serialized and inlined into the page. This ensures consistency when the page hydrates, and saves network round-trips (and also means less data needs to come over the wire, since everything can get compressed using the same gzip dictionary or whatever).

Isomorphic (use the same components server-side and client-side)

This definition is actually a strict subset of the first definition: as the same script must (by definition) run inside both a server/Node.js context, but also a browser DOM context

Universal = code that can run in any JS runtime (browser and/or node). Isomorphic = application that runs the same universal code in multiple runtimes to avoid code duplication.

After that, the app works like an SPA. It's got better SEO because when a search engine bot asks for a page, the server returns the whole rendered HTML page, with content and meta tags.

An isomorphic app is a web app that blends a server-rendered web app with a single-page application.