How Drupal works?

To understand how Drupal works, you need to understand Drupal's page serving mechanism.

In short, all the calls/urls/requests are served by index.php which loads Drupal by including various include files/modules and then calling the appropriate function, defined in module, to serve the request/url.

Here is the extract from the book, Pro Drupal Development, which explains the Drupal's bootstrap process,

The Bootstrap Process

Drupal bootstraps itself on every request by going through a series of bootstrap phases. These phases are defined in bootstrap.inc and proceed as described in the following sections.

Initialize Configuration

This phase populates Drupal’s internal configuration array and establishes the base URL ($base_url) of the site. The settings.php file is parsed via include_once(), and any variable or string overrides established there are applied. See the “Variable Overrides” and “String Overrides” sections of the file sites/all/default/default.settings.php for details.

Early Page Cache

In situations requiring a high level of scalability, a caching system may need to be invoked before a database connection is even attempted. The early page cache phase lets you include (with include()) a PHP file containing a function called page_cache_ fastpath(), which takes over and returns content to the browser. The early page cache is enabled by setting the page_cache_fastpath variable to TRUE, and the file to be included is defined by setting the cache_inc variable to the file’s path. See the chapter on caching for an example.

Initialize Database

During the database phase, the type of database is determined, and an initial connection is made that will be used for database queries.

Hostname/IP-Based Access Control