They used the case interview method. I would recommend preparing by reading books such as Case in Point and familiarizing yourself with the vocabulary and frameworks of case interviews. Try to demonstrate good reasoning, quantitative analysis, and don't forget to be friendly.

two rounds of case interviews, candidate-led, tackling a real client problem through a structured, hypothesis-driven framework. need to balance analytical rigor with a collaborative, team-oriented approach, focusing on delivering clear, actionable, and insightful recommendations