Intonation and stress for Chinese pronunciation
(I'm a native Chinese user.)
Any language has its own pattern of intonation and stress variations over a single sentence or many sentences - this is not exclusive to Chinese. Think about English for example (or any other language you know). I don't think such intonation and stress variations (except for the tones of each Chinese character) should be learned by learning the rules that these variations satisfy (in various contexts), because I think it is incredibly unnatural and inefficient for people to remember these rules, and that these rules are in fact very difficult for people to follow. I think it is [very misleading] to suggest to anyone that they learn these variations through remembering and following such rules, unless the person somehow doesn't have the resources which would allow them to listen to properly spoken Chinese.
My suggestion is, ignore these rules and simply listen to native users (who speak Chinese well - not all of them do in fact) speak and imitate them. The way you learn such intonation and stress variations in Chinese should not be different in any notable way from how you learn such variations in any other language, e.g. English.