How are values passed from the database to the templating system?
The function that does the job is theme_render_template(), which contains the following code.

  // Extract the variables to a local namespace
  extract($variables, EXTR_SKIP);

  // Start output buffering
  ob_start();

  // Include the template file
  include DRUPAL_ROOT . '/' . $template_file;

  // End buffering and return its contents
  return ob_get_clean();


The function is called from theme(), which uses the following code.

// Default render function and extension.
$render_function = 'theme_render_template';
$extension = '.tpl.php';

// The theme engine may use a different extension and a different renderer.
global $theme_engine;
if (isset($theme_engine)) {
  if ($info['type'] != 'module') {
    if (function_exists($theme_engine . '_render_template')) {
      $render_function = $theme_engine . '_render_template';
    }
    $extension_function = $theme_engine . '_extension';
    if (function_exists($extension_function)) {
      $extension = $extension_function();
    }
  }
}

// In some cases, a template implementation may not have had
// template_preprocess() run (for example, if the default implementation is
// a function, but a template overrides that default implementation). In
// these cases, a template should still be able to expect to have access to
// the variables provided by template_preprocess(), so we add them here if
// they don't already exist. We don't want to run template_preprocess()
// twice (it would be inefficient and mess up zebra striping), so we use the
// 'directory' variable to determine if it has already run, which while not
// completely intuitive, is reasonably safe, and allows us to save on the
// overhead of adding some new variable to track that.
if (!isset($variables['directory'])) {
  $default_template_variables = array();
  template_preprocess($default_template_variables, $hook);
  $variables += $default_template_variables;
}

// Render the output using the template file.
$template_file = $info['template'] . $extension;
if (isset($info['path'])) {
  $template_file = $info['path'] . '/' . $template_file;
}
if (variable_get('theme_debug', FALSE)) {
  $output = _theme_render_template_debug($render_function, $template_file, $variables, $extension);
}
else {
  $output = $render_function($template_file, $variables);
}