Why are source files Next.js?
Templates and sections are heavily animated, and heavy animation is exactly what usually breaks SEO. Next.js gives us server-side rendering and bundle separation, so the page stays immersive for people and readable for crawlers.
Immersive pages are usually one big client-side bundle: the bot arrives, finds an empty div, and leaves.
Server-side rendering breaks that trade-off. The markup renders on the server, so a crawler gets a complete document with real headings and copy, while the visitor's browser hydrates it into the full animated experience.
Bundle separation does the rest — 3D, shaders, and large motion code load when they're needed instead of holding up first paint.
Next.js is the most direct route to both. If you ship something else, that's what the prompt path is for — nothing in the design depends on the framework.