Drupal 8 represents a lot of changes and a steep learning curve for many Drupal developers and themers. While many of these changes are exciting, there are many things to learn just to get started. One way to learn about the code involved with Drupal 8 modules and themes is to take a look at core's modules and themes for examples to follow. Another, is to use a code scaffolding tool like Drupal Console to generate boilerplate code and comments that you learn from and then customize.