I use a 2014 Mini as a server to watch my large library of ripped movies on two Apple TV's. It is still running iTunes with Home Sharing under Mojave. You could do the same thing with newer versions of MacOS by turning on Media Sharing in System Preferences. It is accessed via the built-in "Computers" app on the Apple TV.
There's no way to connect a disk to the Apple TV, you need a computer to host the media. There are some NAS devices that support Plex, so that might be an option but I have never really looked into it.