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.
Twitter has been publishing about their architecture and all the updates to it for years and years, and given countless presentations on it. https://blog.twitter.com/engineering/
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.