I know the function to create socket:int socket(int domain, int type, int protocol); is located in #include <sys/socket.h> and I can find it on my linux file system.
But where I can find the implementation of this function? I couldn't find a matching one in the kernel source.
Look on kernel.org for the authentic kernel source. Understand that socket(2) is one of the many syscalls(2) (you need to understand precisely what system calls are) so it is implemented inside the kernel as sys_socket and/or sys_socketcall and/or do_socket; sockets and network code are an entire subsystem (net/) of the kernel, so see its net/socket.c, etc etc... See also socketcall(2)
Application user-side code are simply issuing a syscall, so socket(2) is a thin wrapper around a system call (in GNU libc or musl-libc or whatever implementation of the C standard library you are using). See also this.
sys/socket.h should be in /usr/include. Typically, it's a part of the "GNU C Library", but depending on your system, that might also be a different C library (ie. on systems you can't call GNU/Linux, like Android, there might be different libc than glibc).
However, that's just the header, not the implementation of the syscalls beneath! You will have to look through glibc's source code (which usually is not installed, only the headers), and then match what you find there to the system calls implementation in linux.
if you want to check the linux implementation of socket creation, you. can look here
