Google For Jobs is a Search Engine Results Pages (SERP) feature which will display three job listings when a web user appears to be searching for job openings. This is a SERP Feature that will only be displayed when it is clear that a web user is explicitly looking for jobs to provide the most value.
Google For Jobs compiles these listings from a variety of large job posting websites, such as indeed.com and monster.com. As such, these large job websites are supported by Google, with traffic from search engines directed onto their websites.