In Next.js 15, which of these makes a page dynamically rendered at request time?

Awaiting cookies() from next/headers is correct. Request-time APIs such as cookies(), headers(), searchParams and connection() make the route render dynamically, on each request. The tempting "Calling fetch without any cache options" is wrong. An uncached fetch alone does not make a route dynamic, and the route can still be prerendered at build time.