A process is more than just an address space and a set of threads, it
has metadata.  A simple example is its process id.  Metadata is stored
in the OS, in a structure called a process control block.

An important bit of metadata is a process's "environment."  An
environment is a map from arbitrary key to a string value.  A
forked child process inherits the environmenet of its parent.
So, beyond passing arguments to a child process via argc/argv,
a parent can pass them through the environment.