According to this video here are some tips for studying, understanding and remembering subjects over a long time:

  • Concentrate on one thing at a time: Avoid multitasking and distractions.
  • Recall: Spending a few minutes after finishing e.g. a chapter or a video to recall its 3 key points can vastly improve your understanding
  • Fyenman technique: Explain the subject to someone who does not understand it. Simplify it if using analogies.
  • Spaced repetition: Re-read the same thing every few days, allowing intervals in between. 

As MEdIES may we add, that doing physical exercise during these intervals can improve your comprehension and memory. 

Source: Will Schoder YouTube channel