SPAs run in the browser, therefore they must be written in JavaScript (or something that compiles to it.)
SPAs have the drawback of having to load a bunch of JavaScript before rendering anything, so the initial page load tends to be slow. Next.js gets around this by performing SSR for the initial page view, and then you're already looking at that page while all of the JS loads to render subsequent pages.
They stand for Single Page Application (client side) and Server Side Rendering (which makes a JS app render more like a normal server side page for the client) respectively.
SPAs run in the browser, therefore they must be written in JavaScript (or something that compiles to it.)
SPAs have the drawback of having to load a bunch of JavaScript before rendering anything, so the initial page load tends to be slow. Next.js gets around this by performing SSR for the initial page view, and then you're already looking at that page while all of the JS loads to render subsequent pages.