I always start by asking myself what I need then I craft something then I think about other users

but where find design, where find temlate

This isn't really a help thread about API Routes, App Directory or next/image. Probably fit in "Other"

To answer your question, usually you would get a design from a designer using something like Figma, then the developer builds that into an actual site.

What do you mean templates? Do you just want a preset to use? There are many component libraries you could use.

I really like shadcn/ui and tailwindui.

https://ui.shadcn.com/
https://tailwindui.com/ (tailwindui is not free)