What Is Complexity in a System?

In engineering, usually we tend to look at the service s as a black box. So do modern systems often account for a number of goals? Or is it, i guess thits s kind of easy if yo ave one clear business goal that everything kind of works towards? What what is complex ey look like as systems become complex, or goals become complex? Of course, that's a nosom question. Can give you a quick example. I would actually love to hear that. Yah.