So I've got a Mac Mini on my home network, acting ( in part ) as a media server, using Media Sharing ( enabled via System Preferences -> Sharing ).
This allows me to share my DVD library across my various Apple devices, including iOS, tvOS and ... macOS.
Having confirmed that I can see / play movies etc. from my iPad and from my Apple TV, I wasn't able to see the media from another Mac on the same LAN.
In both cases, the Macs are using the native TV.app - the Mac mini to manage the library and a MacBook Pro to consume the shared media .....
As ever, I was holding it wrong ...
In TV.app, there's a Library tab, but that was only showing my purchased media: -
Thanks to this : -
for helping me out of my own mess!