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]<p>[1] <a href="https:&#x2F;&#x2F;msdn.microsoft.com&#x2F;en-us&#x2F;library&#x2F;windows&#x2F;desktop&#x2F;aa365564%28v=vs.85%29.aspx" rel="nofollow">https:&#x2F;&#x2F;msdn.microsoft.com&#x2F;en-us&#x2F;library&#x2F;windows&#x2F;desktop&#x2F;aa3...</a><p>[2] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Hierarchical_storage_management" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Hierarchical_storage_managemen...</a>