You can use it for anything especially with its new server islands.

https://astro.build/blog/astro-5/#server-islands

So you can mix static rendered along with server side rendering pieces and get the benefits of both.

Also it's frontend framework agnostic and you can mix and match.

So you can have a dynamic react component, with a static Astro component, svelte component all in the same page on theory.

https://docs.astro.build/en/concepts/islands/

Then for backend stuff it kinda depends on how you deploy it. It could just be a single monolithic server or serverless per API route and probably everything in between.