There is no magic interaction between the compiler and OS. It simples compiles to the same machine code (i386) and uses interrupts for system calls. Structs are handled as a memory region with a size based on its members, and the member access is simply an offset into that region.

Current it has lots of limitations such as only int and char, no switch statements etc. The biggest change from default C, is allowing functions in structs and implicit passing a struct to a function if it’s a struct function. You can read more here:

https://github.com/joexbayer/C-Compiler