Why does managing a supplier confuse me after the order is placed?

You can place an order. That part is simple — a request, a confirmation, a delivery date. What confuses you is everything after: the silence between orders, the small delays you don't know whether to flag, the sense that you should be 'managing' something but no one told you what that means in practice. Here is the truth you are avoiding: there is no secret skill. There is only contact, kept regular, whether or not anything is wrong. Most people wait for a problem to reach out. By then the supplier has already decided how much they owe you in patience. You are not confused about the relationship. You have simply never built the habit of tending it before it needs tending. This is fixable in a week, not a year. It requires no course, no certification — only a calendar entry you keep.