My Apple tv app library does not have the same content as my actual library on my pc (Windows 10). How do I synch the two??

The libraries should be the same for content purchased from the iTunes Store [1] [2].

The libraries are intended to differ for local media files added to the library on a computer. Sharing local media files requires Apple Home Sharing. Which is not supported in the Apple TV app on third party devices, but is supported through the Computers app on the Apple TV box.

Do you have the Apple TV box? If so, which model? Identify your Apple TV model - Apple Support

Or do you have the Apple TV app on a smart TV or third party streaming device?

Content from the Apple iTunes Store can be watched in the Apple TV app (or iTunes application) on any of your devices. It is not playable on third party media players outside the Apple TV app, due to DRM.

Content not purchased from/​through Apple is not available in the Apple TV app (except for the Apple TV app on Mac, iPhone, or iPad, where it has slightly more functionality). ‘Allowing’ has little to do with it. That content is not available online to stream through the app on those other devices, so the app can’t access it. ‘Not supported’ is a more accurate way of describing it.

The use of third party media players can be an option for some devices and non-DRM’ed content, but not for iTunes Store titles. What device are we talking about?

Oh if it’s purely music then that changes things. You can have your full library available in the music app as long as you subscribe to either Apple Music or iTunes Match. Just make sure you have it set to sync your library

Use Sync Library to access your music library across your devices - Apple Support (CA)

The Apple TV app on the Apple TV box is only going to be accessing your account and have any purchases associated with your ID. Any local content would be accessed via Home Sharing on the Computers app. If using a 3rd party device then it only has the TV app, for the purchases.

You can use a 3rd party app, like Plex or Infuse, to access local content (but not purchases).