I applied through a recruiter. The process took 4 weeks.
Very heavy on technicals. They will start broad and then will dive deeper. Expect a ton of follow-up questions about every answer. Make sure you know everything about projects you've done and past internships. Know fundamentals of mechanics, stress, manufacturing, etc.
The following metrics were computed from 3 interview experiences for the SpaceX Build Engineering Intern role.
SpaceX's interview process for their Build Engineering Intern roles is incredibly easy as the vast majority of engineers get an offer after going through it.
Candidates reported having very good feelings for SpaceX's Build Engineering Intern interview process.