I made it to the third stage of the process, so passed the aptitude and quantitative HackerRank test, but blew the first interview with HR. The quantitative test was pretty fun, and requires good mathematical problem solving more than hard maths. Memorising simple dice and card probabilities would be advantageous. It also had a data analytics section, where we were provided a long dataset with some nasty errors in it, so brush up on reshaping and cleaning data. Had a nice chat with the HR interviewer. She mentioned they hire from all academic backgrounds for the quant researchers, so if you have an unrelated degree but like maths and are interested, you might as well go for it.