Think of it this way - many people who apply don't have a relevant background. That also means you have to develop business knowledge prioritising the industries that are most common in cases.
1) Retail - make sure you understand key retail metrics (P&L lines, same store sales, revenue per square meter and per person, etc) and how distribution works as well as its metrics (share on shelf, etc)
2) Airlines - read about profitability issues, different routing models, cost structure, industry metric s like load factor and make sure you know all possible additional revenue streams
3) Telecom - I would read industry reports and try to understanf how telecom is transforming with internet penetration and tech innovations
4) Banking - you should know the key products in corporate and retail banking and how they earn money (Interest, commision, transactional)
5) PE / M&A - go through available case examples and read about different PE strategies