I haven't used mathematica but look forward to learning it one day as an indulgence. I code mostly in R, and it's renowned for having some hurdles to jump through in order to get a package published on its primary repository (CRAN), but what that high standard means is you can download almost any library and expect to find extremely well documented functions with examples that can be run with minimal (usually zero) additional data/dependencies. It's a real treat, and I miss it massively when using languages with less rigorous and less uniform approaches to documentation.
https://reference.wolfram.com/language/ref/Flatten.html
Just look at the amount of examples and details for this function. It is like that for every function.