I applied through college or university. I interviewed at MIT
Interview
The process was challenging. Make sure you know what the research group does and can demonstrate that your prior knowledge will be useful to the group. Be courteous to the Professors and always follow up with an email after your interview to reiterate your interest.
I applied online. The process took 2 months. I interviewed at MIT (Cambridge, MA) in Sep 2019
Interview
Virtual hour-long interview with 2 people, then a day-long interview with staff and students. Everyone was very kind and easy to talk to. Took 4 weeks to hear back that they had selected someone else.
Phone call and then on site interview. 1 hour presentation about your previous research and multiple 30 min one on one meetings. The process takes weeks. Then go through HR.