How can SEO (Search Engine Optimization) be implemented in Drupal?

SEO in Drupal is very likely setup using the following on the website: 

https://www.drupal.org/project/metatag - Metadata management and auto created metatags, etc. for content when you don't fill it out.

https://www.drupal.org/project/xmlsitemap - A sitemap of content sent to Google

https://www.drupal.org/project/pathauto - Logical paths and URL's vs node/1 node/2

With these at the basic setup, your site will have page titles, meta information, and logical URLs

Configuring H1's / H2's and format of the site is more dependent on the design, theme, and field order on the page, but can be customized through theme adjustments when required.