Every computer that runs Unity Reflect can act as a server. Each server runs locally on the machine and has it own data storage, which stores the models federated from Revit.
When you begin syncing models from Revit to Unity Reflect, you can push this data to the server of your choice (or even multiple servers across different computers). By default, all servers are private, but you can give access to other users in the privacy settings of a server and manage access right from your Unity project page.
While everything is stored, the data is ephemeral. This means that you will not lose anything if you delete the files or make a mistake (e.g., pushing the wrong data to a project). You can always regenerate the data from Revit.
This server storage is independent of the Viewers, which have a data cache that allows users to view data offline.