Check out the islands architecture info, or search around for info on partial hydration. It's similar to how things worked in the jQuery days--you serve up plain old HTML and have a JS function that runs on page load to attach all the event handlers (onclick, etc.) to your dynamic elements (i.e. 'hydrating' the static HTML into dynamic HTML + JS components). The thing with frameworks like Astro is that they try to automate and manage all of that hydration for you, which is nice because for complex sites with lots of different components it becomes a huge, error prone chore to manually wire things up.

edit: Island architecture background info: https://jasonformat.com/islands-architecture/

Currently you build a static site like you do with Gatsby or Jekyll or any other static site generator. In the future it will support dynamic server rendering.