Online assessments across countries and firms really vary. Fundamentally, it's pretty hard to study for these. My real advice is to find online tests / questions and practice. Even across different tests, the principles are all the same:
- Time management (need to think quick)
- Business knowledge/context (incl key formulas)
- Critical thinking (answers that are very similar to each other...clues "hidden" in the text)
- Ability to interpret data/charts
Think of it as a merge of a case and the GMAT/GRE.
You need to be clear on some key case-related and account formulas (margin, growth, breakeven, etc) as well as be good at critically understanding the question (including nuance to questions) and parsing through complicated text with a fair amount of distraction.
You'll also need to be good at chart/graph reading.
The (old) McKinsey PST, BCG Online Test, Bain SOVA test, etc. are all quite similar so leverage resources across all of them.