I had a similar experience during my physics PhD. Going in to the degree I thought I'd just learn physics more deeply. What I ended up learning is how to learn

I had a similar experience during my physics PhD. Going in to the degree I thought I'd just learn physics more deeply. What I ended up learning is how to learn itself, not because of any course I took but because I spent years surrounded by incredibly smart people who asked the right (often difficult) questions about my work. Eventually I learned to ask myself the right questions about my work. I spent my undergrad learning the theory, and then spent my PhD applying it to real problems, working with actual geniuses, being paid a reasonable salary to do it, and getting to discuss it with external colleagues in nice places. The experience will stay with me forever.