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&#x27;t know if you can do user mode filesystems on those, but there is this: <a href="https:&#x2F;&#x2F;dokan-dev.github.io&#x2F;" rel="nofollow">https:&#x2F;&#x2F;dokan-dev.github.io&#x2F;</a> 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&#x2F;mac) to do than a filesystem.