Indeed. All the foundational common ground between all languages in Unix is made for C. I'm talking about the system call interface. All general purpose languages will have a wrapper for this interface, but the main documentation is made for C.

Besides that, libraries tend to be made in C as well. That's because if it's in C, it's easy to make it available for other languages. Most general purpose languages will have some type of "foreign function interface" to call on C code. That means that the main documentation for these libraries will be for C as well.