Why 5G Security = (3GPP + IT + IoT + CT) Security? To answer this question, we break it down to several questions: What is inside a cellular network? How do mobile phones connect? What is ORAN? What is 5GC? What is MEC? What is private 5G? Why traditional IT firewalls aren't enough? We introduce the architecture of Cellular Networks and discuss how it differs from traditional IT. Then we cover private 5G systems and their unique security challenges, with examples. To wrap up, we discuss a Zero-Day, and show how the private, isolated subnet of a manufacturing plant is thrown open to attackers by the peculiarities of CT networks.