That appears to be an iceberg problem. While the public UI of Twitter is simple and is often used as the basis of a starter project in training courses, there's also likely to be a lot of backend admin tools for things like moderation, advertising, sales, etc (not to mention as others have the scaling: at this level it's not so much a "solved" problem but one that requires constant monitoring, tweaking, and preventing newly-minted CEOs from literally pulling the plug on running servers).