I've googled but couldn't find a easy-to-understand guide
Domain Name System in Wikipedia have nice pictures for easy explanation
As per my understanding, request->registrar->hostingNS->index.php
Slightly wrong. Process of getting content from any resource, referenced by name, include 2 stage
- DNS stage, were client stage when the client knows which IP it will need to connect
- Connection stage, were requested work performed
registrar do nothing in this everyday's job - it's main task is only register (over "glue records") NSes of domain in Up-level DNS-zones
Fixed and simplified workflow will be something as
- client request URL (name)
- DNS-servers over hierarchical tree (up and down) will find DNS-servers, which have information about requested hostname (DNS-hoster from question) from URL and return IP (IPs) as response
- To this IP client will send request for resource. Server (webserver in our case) process request and if requested resource is known to him and exist - return data to client
As you can see only one authoritative DNS (for zone) used, if web-hosting separated from DNS-hosting, NSes of webhoster will be untouched
HTH