how dropbox handles uploads, downloads and sync

dropbox is one of those apps that feels totally simple until you start designing it. you upload a file, it shows up on your other devices. share it, someone else sees it. but every step here has a real engineering question buried in it. 50GB uploads can't just be a single POST. sharing 100k files across users can't just be a list scan. and getting bytes from a virginia data center to a tokyo…