You can see changes made by other collaborators in real-time on an open Word document shared on SharePoint. Is there an event raised for these that you can subscribe to in a JS add-in?
Alternatively, can you subscribe to changes made to a Word document hosted on SharePoint in a .net application?
Edit: I see that you can do the latter at a file level using the SharePoint REST API.
Edit 2: I am pretty sure you cannot subscribe to the real-time collaboration changes programmatically. In addition to using the SharePoint REST API, you could use MS Graph API. I am not sure either provide any sort of granularity about changes within a file though, or just tell you which file has been changed.