As far as I know server rendered pages are preferred for SEO. And while Google can process some JavaScript they only recently switched from Chrome 41 (2015) to 74.

This might be a good starting point for understanding some of the issues:
https://www.onely.com/blog/ultimate-guide-javascript-seo/

A PWA doesn’t need to be an (SSR) SPA (the App Shell Model created this confusion) - it is entirely possible to build a PWA where the page is rendered with EEx - but it’s the responsibility of the JS delivered with those pages to enable the PWA functionality on the browser end:

… that doesn’t automatically imply SPA. PWAs are supposed to extend “Progressive Enhancement”:

1. Content is the foundation
2. Markup is an Enhancement (HTML)
3. Visual Design is an Enhancement (CSS)
4. Interaction is an Enhancement (ECMAScript)
5. Network is an Enhancement (Offline first)

https://love2dev.com/blog/pwa-spa/
https://love2dev.com/blog/yes-fast-food-frameworks-cost-too-much/
https://love2dev.com/blog/large-javascript-frameworks-are-like-fast-food-restaurants/

I believe you will always need a server that will run the Next.js server (this is a Node app).

If you then want an API (Phoenix in this case), you will need to run your Phoenix server in addition to the Next.js server.

From:

def index(conn, _params) do
  component_path = "#{File.cwd!}/assets/js/HelloWorld.js"
  props = %{name: "Revelry"}

  { :safe, helloWorld } = ReactRender.render(component_path, props)

  render(conn, "index.html", helloWorldComponent: helloWorld)
end

And
https://github.com/revelrylabs/elixir_react_render/blob/master/lib/react_render.ex#L92-L110

So it looks to me that Node.js is rendering the React content but the phoenix controller is serving the rendered content - i.e. Node.js is rendering but not serving (but I haven’t looked much deeper in to it).

By the looks of things elixir-nodejs communicates with Node.js processes via ports.