Site links are groups of links that appear under the first or second result in Google.
Site links typically appear for brand name searches and provide users with easy access to deeper pages on a website.
For example if you searched for “Amazon” in Google, you’d be presented with Amazon website and a sub listing featuring site links for Prime Video, Books, Wish List etc.
Site links are based on what pages users visit subsequently when arriving on a homepage.
You can’t opt in for Site links, you can only opt out.