I’m trying to understand web API term, used in MDN and after researches, I realized for defining web APIs, a language called web IDL is getting utilized, so I wonder why these objects referred as web APIs, why don’t they get created by JavaScript itself and why there is a necessity to utilize a language as web IDL?

and another thing is, is web IDL only meant for bringing C++ into JavaScript with its own special semantics or it can turn any other language than C++, into JavaScript?

last part was based on text:

Web IDL exposes an interface to JavaScript, which is implemented in C++. Thus its semantics bridge these two languages, though it is not identical to either. Web IDL’s semantics are much closer to C++ than to JavaScript – in practice, it is a relatively thin abstraction layer over C++. Thus C++ implementations are quite close to the IDL spec, though the resulting interface is somewhat unnatural from a JavaScript perspective: it behaves differently from normal JavaScript libraries.