Do you find that the ArduinoJson Assistant is awesome?
Well, check out the ArduinoJson ebook. It's a deep-dive approach into the library and its implementation. In particular, it explains what StaticJsonBuffer and DynamicJsonBuffer are, and how to use them properly.
The book begins with a quick C++ course to catch up with concepts like pointers, references, stack, heap, and others.
It's a "pay-as-you-want" deal, so you can get the book for free. However, you can take this opportunity to thank the author of the library and encourage the development of high-quality open-source software.