If I publish a article/blog, then If I search the article in google search, is the date coming in google search result taken from meta tag or anywhere in the body of the HTML or last-modified-by HTTP header or from sitemap.xml
You find these types of date indications typically on sites that are highly popular, trendy, and generate a fair amount of content daily though not exclusively. These come from the inception date, the date Google discovers the page. How these can be reliably used by Google is if Google is fetching new content daily. The site has to be highly fresh with new and updated content. You will notice your examples are prime examples of such a site. Others would be MOZ and SearchEngineLand.