This guide continues with the Hello World module, showing you how to add custom blocks (and more) step-by-step.
For the truly adventurous, you can skip to the Examples module. The Examples module is the unabridged version of the code that is used in this guide. If it's your first time here, keep reading this guide and check out the Examples modules when you're ready to explore more.
To explore all the truly awesome code and inline documentation contributed by the Drupal community, try http://cgit.drupalcode.org/examples/tree/page_example?h=8.x-1.x.
Also be sure to check out the relevant API documentation: