Wdym? GeForceNOW uses common platform launchers (Steam, Epic, EA app, Xbox app) but modified to allow only authenticate and play that game. Eg: steam only show that game in library, epic/xbox app start directly the game with login screen and ea app automatically start the game too.
You don’t have any contact to any machine resouce like files list/file explorer/browser/task manager, nothing.
Steam games like Cities skylines that uses steam workshop, cannot use steam workshop items, they just don’t download (are forced to don’t download workshop).
If you mean uses Steam Remote Play feature to play in cloud don’t make sense because it is still hosted on your PC. In my case, i’m low-end PC, cannot host an OW2 session to play in myself PC.