Start with on-campus interview, which was more general and more about spreading the message that 3M does Rapid Recruiting every December (so wait for that). Later got a phone call asking me to apply for the online position starting with "RR" (for Rapid Recruiting), and got the on-site invitation after two days. The on-site interview was very fun, well organized and very impressive. It somewhat feels like a conference since I got to meet other candidates (non-competitive) and listen to their presentations. Got the offer one day after the interview.

Interview consisted of a research talk, during which they would ask technical questions regarding the information presented. At the end, they asked more questions and then asked if I had any questions.