Date: Wednesday, 26.February.2014, 0:22 AM | Messge # 3,
Everything else is ultimately only details -- significant details, but variations on that theme. index.php is the control, the menu system determins what the "existing page" is, and tons of occasions get fired in the process of building that page. Plugin modules can hook in to those occasions and modify the workflow/supply additional info/etc. That is also a part of the reason a lot of Drupal resources center on making modules. Without modules, Drupal does not really DO anything other than say, 'Someone asked to get a page! Does it exist? No? OK, I'll serve up a 404.'