I was an intern for 3 summers, so I have applied and been accepted through this process 3 time. Each year, I applied online, and quite soon after (within a week), I received an email from a prospective mentor, who will ask to schedule a phone interview. The phone call is very casual, not very technical in nature. The mentor will talk about themselves and about the internship project. There were mostly questions about my background, interests, and about some of the projects I worked on. Know your projects really well, and ask a lot of questions about the internship project! Afterwards, I would send them a small code portfolio that is related to the internship project. After this initial interview, I always got the offer within 0-3 days.