> In React, using JSX means you can create components and mount them on top of the existing elements on the page.

I am not sure this analysis is entirely correct.

In case of server-side-rendered React pages, React takes the html that arrives from the server and makes it interactive ("hydrates" it, according to the common React community parlance), provided that that DOM generated from that HTML corresponds to the virtual DOM that React generates. It doesn't throw the original DOM away.