From a first look on Windows, this is implemented with NTFS sparse files [1] and a file system minifilter. The sparse files act as placeholders so that you can browse your entire Dropbox structure. When you access a file, the minifilter sees that and starts fetching the data from Dropbox servers in the background. You can think of it as HSM [2]

[1] https://msdn.microsoft.com/en-us/library/windows/desktop/aa3...

[2] https://en.wikipedia.org/wiki/Hierarchical_storage_managemen...

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.

It can't. This is the type of features that have to be developed for each OS separately.