You’re not a “microwave expert”. Claiming to be one would imply that you do understand the inner workings. I write code in Java everyday for my job, but I wouldn’t claim to be a “Java expert” because I don’t have exceptionally deep knowledge of its inner workings.

Usage of a tool does not make someone an expert of a tool. An expert can describe, at least at a high-level, why the tool works the way it does.