Are Unix/Linux system calls all or mostly in POSIX?
Many Linux/Unix programming books say that POSIX library functions may be wrappers of OS system calls, or may be not. E.g. http://www.makelinux.net/books/lkd2/ch05lev1sec1, and https://www.safaribooksonline.com/library/view/understanding-the-linux/0596005652/ch10s01.html
A part (called the Single UNIX Specification) of POSIX defines UNIX. Therefore I think POSIX defines the system calls of Unix (and of Linux).
Then are Unix/Linux system calls part of POSIX library functions?
Linux system calls are listed in syscalls(2). Most are POSIX, but some are specific to Linux (e.g. signalfd(2), etc...). POSIX is a specification and does not "know" about syscalls (which, in the POSIX view, are an implementation detail).