My guess would be lots of distributed queues, some resources dedicated to heavy lifting of few vs random noise of everyone else, and storage layer which allows lots of append traffic without blocking. Maybe some storage special casing like "randos get their tweets serialised on your timeline, but because you're following Taylor Swift we'll just query that common feed in parallel rather than copying it".

They've got lots of engineering blog content, there may be a better answer here https://blog.twitter.com/engineering/en_us/topics/infrastruc...