@x23piracy a windows service can impersonate one of the logged in users without their password. Essentially the service grabs a process running as the user you wish to impersonate, and obtains a handle on the process’s access token (OpenProcessToken). The service then needs to duplicate the token so it can “impersonate” the user without affecting the process we lifted the token from (DuplicateToken), activate that token, and then perform the work. Finally the token handle must be closed.
Its a pretty win32 api heavy approach. The client does not use this technique as there is no guarantee a user is logged in at any given point.
TL;DR: grab an existing process running as that user, and perform multiple win32 API calls to impersonate that user.