Well it is doesn’t have to be only React and Vue. It can be any of these : React, Preact, Svelte, Vue, and SolidJS.

With Astro Islands, we can write our favourite framework just like we do in that framework (mostly as components). Then import it into any of Astro’s front-facing files and put it there.

When we are building our app Astro will parse that component, build the HTML and CSS out of it. By this, all the Javascript is stripped out and we still wrote our code in our favourite framework. When that component hits the client, it just HTML and CSS, hence improving performance by miles.

But what if that was a Interactive or Dynamic component? Astro got that covered too. Astro has developed client directives, which can be added to a framework component to mark them as interactive. Once it is marked, Astro will chunk the Javascript (needed only for that component) while building the app. Once the HTML of that component gets rendered on client, Astro hydrates the component by sending the chunked-out Javascript. In code it looks something like this:

<MyReactComponent client:load /> // with client directive
<MyReactComponent /> // only HTML and CSS

By the same way, we can have multiple components written in different frameworks, sitting at a single Astro page. This is what docs from Astro has to say about it :

An island always runs in isolation from other islands on the page, and multiple islands can exist on a page. Islands can still share state and communicate with each other, even though they run in different component contexts.