Next.js currently uses 3 specific functions to work on Netlify. These are the functions:
Next.js SSR handler: This function handles the SSR pages, API routes and preview mode functionality.
Next.js ISR handler: This one handles ISR routes.
next/image handler: This one handles the Next.js Image component.
An important note about the SSR handler function, because of the way Netlify Redirects work, it also executes for every request that could end up being a 404.
You can check your function logs here: Netlify App. Each function invocation is counted in the requests you see on that usage page.