Google Scholars shows the number “cited by”. These references are automatically extracted from the full text of the indexed items. But be aware: the indexed items include preprint repositories which include not yet peer reviewed or not scholarly articles. For Google Scholar these citations all count the same. You can see the list of citing articles by clicking the link Cited by.
To create a citation alert and receive a notification when a certain article has been cited, search the article and click the link Cited by. You find the button Create alert on the left side of the screen.