Learnings- the way to retain
I had studied probability and statistics (a little more seriously) as a Phd student. Since then I used the concept sparingly.
13 years later, I recall so little about the same; limited to the basic concept. The only instance of using it explicitly came when I was to forecast the lifecycle of batteries being manufactured.
In practical world, you are expected to offer solutions within hours. There is little time to revisit your past learning, demand for additional data from stakeholders, and submit your workings. Though, implicitly the past learning of the concepts does help and shape your decision-making most of times; exceptions being when impulsive.
Hence, the learning from my past is that you are not an expert on a subject till you can teach or practice. We may not get to practice but we can always teach to retain what is learned.