Remix is like the Next.js Pages Router + a helper similar to Server Actions. It does not support Server Components (what the App Router uses) and they're exploring using RSC in the future. So definitely different.
For example, `getServerSideProps` in Next.js is the same as a `loader` in Remix.
Astro probably, if they want to keep using React components.
Otherwise they might also be looking at Nuxt, Sveltekit or Solidstart, but if they're switching top-level frameworks they may as well use Astro because they can mix and match React/Vue/Svelte/Solid