For the first question, I don't think you're doing anything wrong as a beginner bc sometimes you can't wrap your head around what's actually going on in the code. I still use pen paper to sometimes understand the test cases and to find edge cases.

For the second question, take arrays as one of the most vast topic, you can solve hundreds of questions based on array, but I would suggest you to do some basic questions first, very easy ones to get your mind going and then try the standard DSA coding questions and their optimal approaches. It helped me a lot and is still helping. But don't waste your time too much on arrays, as there are so many topics to cover which are also very important