To get you started - from the OPEN(2) man page (
man -S2 open) :
If the file does not exist it will be created. The owner (user
ID) of the file is set to the effective user ID of the process.
The group ownership (group ID) is set either to the effective
group ID of the process or to the group ID of the parent direc‐
tory (depending on filesystem type and mount options, and the
mode of the parent directory, see the mount options bsdgroups
and sysvgroups described in mount(8)).
mode specifies the permissions to use in case a new file is cre‐
ated. This argument must be supplied when O_CREAT is specified
in flags; if O_CREAT is not specified, then mode is ignored.
The effective permissions are modified by the process's umask in
the usual way: The permissions of the created file are
(mode & ~umask). Note that this mode applies only to future
accesses of the newly created file; the open() call that creates
a read-only file may well return a read/write file descriptor.
touch command to create a new file, you'll see the mode passed to
open() is 0666 (i.e.
-rw-rw-rw-). The umask mask will be applied to that mode.
$ strace -e open touch my-new-file
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
open("my-new-file", O_WRONLY|O_CREAT|O_NOCTTY|O_NONBLOCK, 0666) = 3
+++ exited with 0 +++