How CiviCRM works when a url hits
I think you are looking for CiviCRM documentation

There you will find:


CiviCRM developer guide - probably the most relevant place to start for you
CiviCRM administrator guide
CiviCRM user guide


In the Developer Guide you'll find pages like Codebase &amp; Architecture which are a good starting point for your initial question.

CiviCRM Stack Exchange (this site!) is a great place to ask specific questions, and you should also join the CiviCRM community over on CiviCRM chat.