When I finished my engineering degree I spent a few months volunteering as a teaching assistant for senior high school math and physics. I found the material unbelievably easy compared to how I remembered it being as a student. When I was a student, I always thought nothing was really being retained, but each time you push yourself to a higher level in studying, your “base” knowledge moves forward. I think we all get ourselves into a state of hyper-retention for exam day, and then “forget” the most bleeding-edge stuff, but still retain quite a bit (more than we think… I “think”).
All the same, it still seems like a mountain to climb when studying begins.