The people here are so incredibly friendly! It did take a long time between my initial application until the first phone screen, but the recruiters generally go by school so it probably means your school hasn't necessarily been reached yet.
1) Phone screen, purely behavioral: you don't have to have used MongoDB before (most people who get intern offers haven't), but definitely show an interest and indicate you did your research
2) Technical phone interview: this is in part behavioral, but there are some technical questions mixed in. My interviewer asked me one algorithmic problem that had 0 coding, just description of pseudocode, and then another algorithmic problem that we solved on Coderpad. Both questions were totally reasonable in my opinion, not more than LC easy.
3) On-Site: You have lunch with the New Grads and have three technical interviews (although they say one is more behavioral, you get technical questions to some degree in that one as well). None of the questions were unreasonably hard, LC should have you more than prepared. The questions also were generally not really LC style quizzing, it was more like a collaborative exercise than anything. They are really good at giving hints to nudge you in the right direction if you feel a bit lost, so definitely capitalize on that if you can.
In general, the recruiters and the interviewers were consistently extremely polite, forthcoming, and seemed happy to be working there. The company seems ethical as a whole and takes people's needs strongly into account. They pride themselves a lot on culture, so make sure you go on site if possible to experience it firsthand (will be reimbursed)! All the interviewers were clearly smart, amicable people. When I received my offer letter, a bunch of my interviewers immediately emailed me to say congrats and make themselves available for any questions, which was not the case for almost all the other companies I received offers for this season.