How should one approach NCERT's Microeconomics and Macroeconomics? Do both of them need selective reading or a comprehensive one? How should I proceed after their completion and, what Mrunal economics videos are to be followed?

After going through your question I presume that you are looking for economics guidance regarding the competitive examinations. In this particular scenario especially for economics, the NCERT is for both microeconomics and macroeconomics must be thoroughly done. In other words, you must study both the books word by word.

On the other hand, the general topics in microeconomics are welfare, demand, supply, elasticity, tax incidence. And the general topics for microeconomics our national income, various multiplier related concepts, the balance of payments, budget, money, and banking. Getting additional help and guidance through various online platforms is personal opinion and choice.

Tip - keep the material short and revise it several times.