Traditionally we think of them as being caused by differences in land-sea heating.
But more recently, some researchers, like Simona Bordoni and William Boos, have argued otherwise.
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Thats a very heavy question and might need a dozen pages to briefly answer, but however its better to summarize the answer and refer to the most detailed review work of Pin Xian Wang and his team entitled "The global monsoon across time scales: Mechanisms and outstanding issues".
I hope this can answer all the questions anyone have in mind. General definition can be that (Global Monsoon (GM) as a planetary scale circulation system. The primary driver of the GM is solar insolation, and the specific features in the underlying surface, such as land-sea distribution, topography, and oceanic circulations, are mainly responsible for the differences among regional monsoon systems.) derived from the above work.
The most important point to be considered that the paper listed above is based on CMIP5 models, however many authors in the region have documented the deviations and differences in all CMIP5 models globally. I will also suggest the work of Alexis Berg for African monsoon system.