A system call makes the switch to kernel mode via a trap, which is essentially a software interrupt.

A system call is the only way in which a mode switch from user mode to kernel mode can occur.

A system call always causes a context (process) switch.

In our programs we use pcntl_fork  or os.fork() to create a new process. This is the actual system call  and we pass it directly to the kernel for service.

The arguments that users pass to system calls do not need to be examined by the OS - this is the job of the calling programming language.