The basic theory can be seen in the single-respected Olympiad, three-volume yellow book, which is a bit old, but there are too few contents to be tested in the mathematics competition, which will not change much;
It is also necessary to prepare a standard question bank, including at least all the topics of the national league, winter camp and IMO over the years, such as the "Olympic Mathematical Inscription" published by Nanjing Normal University.
The rest depends on how far you want to go. If you want to join the national team, you have to do more high-level topics. For example, the Chinese school mathematics textbook, the course of Olympic Mathematics (published by Hunan Normal University), the collection of Olympic Mathematics 1994, the training team questions over the years, IMO pre-selection topics and so on. Anyway, the math contest is basically a process of accumulation, except for a few talented exceptions, which are all based on experience.