There was a test assessment in HackerRank. Positive feedback was sent the next day and the second step was an interview with a member of the team about one hour long.

Same as what they said in their career page: stage 1, online assessment; stage 2, codepair and math interview; stage 3, interview with several team members. You'll have a two hours interview at the final stage. Maths questions are always probability, no others.