How do I take notes if what matters more is the topic questions?

I spend hours take notes using my prof's PDF slides, but I don't really need to note down everything, I only need a few notes to use as reference when I'm doing exercises on whatever lesson/chapter I'm learning.

The problem is, I don't know which things to take notes of exactly, like which keypoints would I need to remember in order to solve those exercises. Basically like a paradox.

So how do you guys take minimal notes to spend more time actually doing practice questions?

Also, one more question (a little off topic), do you need to understand each chapter/lesson deeply or is just shallow understanding enough (and you keep strengthening your understanding as you do more exercises).