You have a client-side web application. You want the initial-state HTML to come over the wire on the first response rather than wait for the client application to load, make a request, and then render the result.
So you use a tool like those listed in TFA (like Next.js) to run your front-end app logic on the server to generate the HTML and send it to your client where your front-end app mounts to that HTML and takes over (cutely named rehydration).
This is relatively cutting edge stuff which is why it's weird to assume OP is an idiot who never knew about PHP or CGI when he wrote an article that is obviously talking about client-side applications and rendering them on the server.
You have a client-side web application. You want the initial-state HTML to come over the wire on the first response rather than wait for the client application to load, make a request, and then render the result.
So you use a tool like those listed in TFA (like Next.js) to run your front-end app logic on the server to generate the HTML and send it to your client where your front-end app mounts to that HTML and takes over (cutely named rehydration).
This is relatively cutting edge stuff which is why it's weird to assume OP is an idiot who never knew about PHP or CGI when he wrote an article that is obviously talking about client-side applications and rendering them on the server.