Most forms in Q2A post back to the same page you're on. For example this answer form I'm typing in posts to "../55224/how-to-know-the-location-of-the-form-php-action-file"
The "action" files that Q2A uses are in the qa-include/pages/ folder. Most should be fairly self-explanatory - typically plurals (tags.php, users.php) are for list pages, singular (tag.php, user.php) are for individual pages.
In each PHP file there are various "branches" of logic for whether we're just displying a page or if someone submitted a form back to the page.
Note: this may change in the near future to separate out the logic a bit more.
