static inline void ed25519_randombytes(unsigned char *x,unsigned long long xlen)
{
  int i;

  if (ed25519_random_fd == -1) {
    for (;;) {
      ed25519_random_fd = open("/dev/urandom",O_RDONLY);
      if (ed25519_random_fd != -1) break;
      sleep(1);
    }
  }

  while (xlen > 0) {
    if (xlen < 1048576) i = xlen; else i = 1048576;

    i = read(ed25519_random_fd,x,i);
    if (i < 1) {
      sleep(1);
      continue;
    }

    x += i;
    xlen -= i;
  }
}

As you can see it uses this fd here. I am really interested how it works when compiled to wasm because this is a sys call ed25519_random_fd = open(“/dev/urandom”,O_RDONLY);

when I compiled it using emcc and executed in inside node or browser, it works, it produces random bytes. But not really sure how.