The official training (all of it from content to developer) shows how to use the the basics of content entry to the more advanced content editor portions. It also shows the standard development practices within the Kentico UI as well as some of the more advanced examples of what could possibly be done with some custom code.
Yes there is not a lot of examples for things you or others may think are basic tasks but if you really think about them, they are specific to your own site. In the time I've worked with Kentico, documentation has been significantly improved and Kudos to Kentico for this. If you're looking for something specific, you might need to Google around or as others have suggested, read the documentation more in depth to get specifically what you need.
Kentico won't provide specific examples on how to implement a slider or how to fully create a custom payment provider or how to implement a custom tax provider because once you get in past creating the standard boilerplate code, it gets very specific to the plugin or 3rd party provider you're using.
So maybe instead of being frustrated, document out what you need from a user perspective or high level and then research what Kentico has to help you accomplish this. Using the system more and more will only help you become more familiar with it. I've been using Kentico since 2008 and still learning new things every day. Others have been using it since it came out in 2004 and still using methods or approaches from 2004 while others have educated themselves and are using the modern approaches for site implementations.
What I'm trying to point out is, Kentico documentation is not here to provide every possible scenario for you on your site implementation. Documentation is provided to help educate you with the tools Kentico offers and in some cases provide generic examples on how the tools may be used.
Good luck and hope your frustration goes away soon!