Google adds them through its own secret automated algorithms.If you have a Google Ad words program you can create campaign and ad group level site links. Google only show site links for results when we think they'll be useful to the user. If the structure of your site doesn't allow google's algorithm to find good site links, or Google don't think that the site links for your site are relevant for the user's query, Google won't show them. At the moment, site links are automated.

Sitelinks are hyperlinks to website subpages that appear under certain Google listings in order to help users navigate the site. The site owner cannot add any sitelinks; Google adds them through its own secret automated algorithms.If you have a Google Adwords program you can create campaign and adgroup level sitelinks. Sitelinks shown when the structure of the site follow the google guidelines or algorithm

Build more internal links to the pages you want to appear as sitelinks. In other words, the pages that have many internal links pointing to them, are likely to appear as sitelinks.

Under Search Appearance, click Sitelinks.
In the 'for this search result box' complete the URL for which you don't want a specific sitelink URL to appear.

Make sure that your website's name is unique. ...
Add structured data to your Website. ...
Ensure your website's structure and navigation is crystal clear. ...
Rank #1 for your brand name in search results. ...
Add a sitemap.

Useful information!
Thank you very much for sharing this thread post.