My guess is this would be a very high level look at their architecture and maybe a closer look as some specific portions. Like you said these systems are so complex that you wouldn't be able to figure out what is even happening in a day.

I don't agree that it's to drive feat at Twitter, as just about any engineer will know what I just said. What I think it's about is driving the headlines, "Tesla engineers are doing a code review at twitter" sounds very important to the lay person.