TYPO3 Core

Source says:
// get root template
$rootTemplates = $GLOBALS['TYPO3_DB']->exec_SELECTgetRows('pid', 'sys_template', 'deleted=0 AND hidden=0 AND root=1', '', '', '1');
if (count($rootTemplates) > 0) {
return $rootTemplates⁰['pid'];
}

Siteroot is searched - but if there is more than one it's not deterministic which will be the "first" one. Since we don't have a "sorting" in sys_template, at least sort by crdate to use the first siteroot that was created.