Expert Next.js development is largely about placing computation in the correct environment.

Use the server for database access, secrets, heavy data preparation, authorization, and initial content. Use the browser for interactions, local state, animation, immediate UI responses, and browser APIs.

There is no universal rule that all data must be fetched on the server or that all interactive applications should use client-side fetching. The correct choice depends on latency, freshness, interaction requirements, SEO, security, and caching.