The PX4 firmware already includes a few example applications, some of which are similar to the examples created for HoverGames. They can be found under the src/examples folder and should be included by default on most builds.
The PX4 Developer Guide also includes a "getting started" page for writing your first PX4 module. This is also based on some of these examples. It explains some of the basics of creating a new module, building your new code and interacting with it from the console. It also gives an introduction to using uORB, the middleware layer that allows you to communicate with other modules.
It is strongly recommended that you start with these examples and then slowly start exploring other parts of the software. The PX4 Developer Guide is a good place to get information and you can always ask your questions on the PX4 Discuss or Slack.
Example code for add-on hardware
Example software for add-on components used in the HoverGames is available on their own pages. You can find them by scrolling down in the menu on the left. They are listed under "Add-On Hardware".