The source code for process_create/3 is located in file process.pl
The source code for process_create/2 is located in file process.c
Details
process_create/3 is imported using
:- use_module(library(process)).
The source code for process_create/3 and process_create/2 is not in the SWI-Prolog library folder on GitHub as one might expect but in the package clib.
process_create/3 is a Prolog wrapper around process_create/2 which is C.
clib - Assorted external libraries: processes, sockets, MIME, CGI, etc.