This insecruity stems from a lack of precision and focus in almost all case preparation books on the market.
Here is an ABSOLUTELY CENTRAL notion you must understand:
Structuring a case does NOT mean to give just the interviewer a list of all the "buckets" you're going to look at! This would not be a proper structure - but just a bucket list... and a pretty random one most of the time!
Structuring a case means to explain to the interviewer the LOGIC according to which you will answer the question at hand. "Areas to look at" are just a byproduct of this logic.
Therefore it is not very helpful to start with defining qualitative "buckets" to look at (e.g., "I'm going to start with looking at the market, then I want to understand the customers etc."). This is, at the end of the day, quite random, with no clear logic from which your areas are derived (other than "experience" or "gut feeling", both of which are not what MBB interviewers are supposed to assess!).
If you think about it, this approach is the opposite of how you should work as a strategy consultant! Defining buckets and then hoping to find something interesting in there is pure explorative working - another word for this is "guessing".
One fundamental thing that needs to be learned in order to rigorously disaggregate the value drivers of a business and, hence, business-related questions, is how to set up rigorous driver trees. The driver tree allows you to identify the numerical drivers and sub-drivers of your focus metric (e.g., profits). The qualitative elements (such as consumer demand, market structure, company operations, etc.) then have to be mapped to the sub-branches of this tree!
Hence, your analysis has two steps. Imagine you want to run a diagnostic on why profits have fallen: