My first guess is decoupled drivers with an abstraction that the actual dropbox client service uses. One driver for windows, one for mac. Though I don't know if you can do user mode filesystems on those, but there is this: https://dokan-dev.github.io/ Or else they may have used some fs event notification library to intercept access to files, but I think that would be harder (impossible? IDK win32/mac) to do than a filesystem.