Honestly, getting out of classroom environments where memorization was held as a priority over comprehension. There was no single technique over all.

So, I went form a D/C/B student in K-12 to a B/A student as an undergrad to a solid A student when pursuing both my masters degrees.

Age also played a factor to it. But one key thing has been constant from K-12 to now... if someone demands rapid fire regurgitation of memorized bs I will do poorly, or as an adult I will just walk away knowing that. Want a thorough exploration of key concept and causal relationships etc? An essay? That's my jam.

Also, there is "studying" to pass a test to get a grade, and then there is studying to learn and understand something. Most of the stuff I studied to pass a test I have not retained at all where as the stuff that was studied for sake of comprehension has stuck around by far better. Which is also at the core of the above issues on top of if one does not have any interest in a topic its likely not going to be retained for very long whereas being passionate about, or even slightly interested in a topic retention, comprehension and testing is improved a great deal.