I had three interviews in total: 1. HR call, 2. First round case study with manager, 3. Second round case study with partner.
Interview content: All interviews were very comprehensive. HR interview felt a lot more like an actual interview rather than a chat - she asked a lot of specific questions about experience, profile, willingness to travel, etc. Case studies were both rather tricky and in heavy industries, both tested your pragmatic thinking about approaches and solutions (vs. theoretical). There was some math, which was easy, but the cases were really aimed at understanding the candidate's ability to understand complex o&g/mining operations, which is fair since that is what they mainly do. They also spent a decent amount of time on small talk and fit part, which meant they really want to get to know you as a person.
Interviewers: all were incredibly friendly, you could tell the culture is great, all really work with you to help you understand the case, very very professional.
I did have a couple of issues though, 1. The recruiter who scheduled the interviews was a bit disorganized, she sent me wrong dates and interviewer names couple of times, it was unclear which round I was in (2nd interview of first round or first interview of second); 2. at the very end, after my rejection, they offered feedback but never followed up to actually provide it; 3. I think they are looking for a certain profile (i.e. willingness to travel so no family ties, ability to be on site and speak to engineers, etc.) which should be filtered out at the screening stage.