How nextjs trigger a request for next page content when click Link?

[AlanKnightly]

I was new to nextjs and have a puzzle about how nextjs trigger a http call for content of next page?
I read the source code of Link and router but have more question...

1. in router component I can't find where the router events defiend,e.g. routeChangeComplete:
   https://github.com/zeit/next.js/blob/8d024554a7f1d335016e08ea9739290fe5d6ce3a/packages/next/next-server/lib/router/router.ts#L496
2. And I wonder if the http request for content of next page is just excuted in the router event?