Didn't make it to all steps. I just went through the first step which was a phone call with a recruiter. It was a good interview, I just wasn't prepared at the time.

First round was an hour-long phone call with an engineer, telling me about various roles they had open and asking me some basic questions about my resume and experience. Very low-key and friendly. Second round was much more rigorous and rigid. 4-hour session in total. It started with an initial presentation from an engineer talking about their projects, LLNL work, etc. Moved on to a technical presentation for 30 minutes where I presented a project I helped lead to a group of a dozen engineers or so, with Q&A after. Got a quick break, then jumped into two back-to-back panel interviews where they had in-depth questions about my past experience, mostly technical. Finished with a short session where I got the chance to ask my questions, along with a benefits summary presentation. Word of warning - be as prepared as possible for the technical presentation. They will grill you on the fundamentals and details of whatever project you're sharing. This seemed like the most important part of the interview.