Extbase controller v12

$this->currentRequest->getAttribute('routing')->getPageId();

or:

$uriBuilder = GeneralUtility::makeInstance(UriBuilder::class);
$uriBuilder->getRequest()->getAttribute('routing')->getPageId();

Extbase controller v10

Access the current page's ID and content element ID in an exbase controller action:

$contentObj = $this->configurationManager->getContentObject();
$pageUid = $contentObj->data['pid'];
$contentElementUid = $contentObj->data['uid'];

Fluid

With vhs:

{v:page.info(field: 'uid')}