First, there is an online assessment that tests coding skills. After that, you have to do a general knowledge assessment. Then you actually get interviews in which they ask questions about coding and problem solving.

Applied online. First round was a series of aptitude tests. Based in a business context, simple arithmetic and logic exercises. Second round was a coding assessment. Equivalent to a harder medium-level Leetcode problem. Third round was then an online zoom interview. Asked about experience with teams, greatest weakness, what do you know about optiver, what motivates you, why do you want to work at optiver etc.