Using Rails? Read Rails documentation.
Using bare Ruby? Read Ruby documentation.
And literally every programming language is like this. C# won't contain Unity C# classes. Basic Python won't have Numpy classes. JS won't have React functions. And so on. Also it's not like Ruby is the only language to ever have monkey patching...
It's absurd that this is being brought up as a Ruby weakness when both the Ruby website and the Rails website each have amazing documentation and if you actually read the documentation, go through tutorials, it's all laid out very clearly.
https://www.ruby-lang.org/en/documentation/
Dunno, whenever I learn a new language, I read the official docs. When I learn a new framework, I read the official docs. Even when I was an absolute newbie, I learned from the resources on the official website.
It can be one of the most pleasant languages to read, but a lot of hidden knowledge is required to write it like Ruby wants you to.