For me it was while studying for my license. I have a much longer past in electronic circuits than in radio, so for me that’s the component of our hobby that attracted me the most. But I’ve always only messed with DC circuits, never AC. Not necessarily by design, just that a lot of hobby circuits are DC (for good reason). So studying for the test caused me to learn a significant amount about electricity. But now I hear people at work talking about reactance or inductance or whatever all the time. And truth be told it was probably happening before, too, but I never noticed it because these concepts weren’t on my radar.