Most Unix systems are fundamentally very similar. There are variations between different machines, and variations in what facilities are provided by the kernel. But the interface to the operating system facilities is, for the most part, pretty uniform and simple.
The code for Unix systems is in the directory unix, at the top level of the sysdeps hierarchy. This directory contains subdirectories (and subdirectory trees) for various Unix variants.
The functions which are system calls in most Unix systems are implemented in assembly code, which is generated automatically from specifications in files named syscalls.list. There are several such files, one in sysdeps/unix and others in its subdirectories. Some special system calls are implemented in files that are named with a suffix of ‘.S’; for example, _exit.S. Files ending in ‘.S’ are run through the C preprocessor before being fed to the assembler.
These files all use a set of macros that should be defined in sysdep.h. The sysdep.h file in sysdeps/unix partially defines them; a sysdep.h file in another directory must finish defining them for the particular machine and operating system variant. See sysdeps/unix/sysdep.h and the machine-specific sysdep.h implementations to see what these macros are and what they should do.
The system-specific makefile for the unix directory (sysdeps/unix/Makefile) gives rules to generate several files from the Unix system you are building the library on (which is assumed to be the target system you are building the library for). All the generated files are put in the directory where the object files are kept; they should not affect the source tree itself. The files generated are ioctls.h, errnos.h, sys/param.h, and errlist.c (for the stdio section of the library).