Could someone explain the magic behind --tty to me?

[Ninlives]

I got lost in the source code. I'm experiment with namespaces, and when I exec systemd inside my wrapper program, the output is not printed to stdout.
I tried using openpty, this is what I was doing so far:

int pty;
int tty;
char pty_node[256];
if (openpty(&pty, &tty, pty_node, NULL, NULL))
	exit_with_error("openpty");

bind_console(pty_node);

pid_t pid = fork();

char buffer[1024];
ssize_t read_count;
int status;

switch (pid) {
  case -1:
    exit_with_error("fork");
  case 0:
    if(setsid() < 0)
    	exit_with_error("setsid");
    if (ioctl(tty, TIOCSCTTY, 1))
    	exit_with_error("ioctl");
    // exec systemd here
    if (execvp(argv[1], argv + 1))
    	exit_with_error("execvp");
  default:
    while ((read_count = read(pty, buffer, 1024)) > 0) {
    	char *ptr = buffer;
	size_t ptr_len = (size_t) read_count;
	
	while (ptr_len > 0) {
	  ssize_t write_count = write(STDOUT_FILENO, ptr, ptr_len);
	  if (write_count < 0)
	  	exit_with_error("write to stdout");
	  ptr += write_count;
	  ptr_len -= write_count;
	}
    }

    if (read_count < 0)
    	exit_with_error("read from pty");

    rc = waitpid(pid, &status, 0);
    printf("%d status: %d\n", pid, status);
}

bind_console links /dev/console with pty_node returned from openpty. Using this approach I can see the output only if I set ForwardToConsole=yes and TTYPath=/dev/console in /etc/systemd/journald.conf, and I failed to see the login prompt.

I'm wondering how does podman handle this? podman could capture the output of systemd and show login prompt using --tty without configuring the journald.conf, what's the trick behind the scene? What am I missing here? Thanks!