Years ago, after Xbox live for the original Xbox shut down, there was a program you could run on your PC that would connect to others running that software.
It would emulate a local Xbox and do matchmaking and your local Xbox would see the others as “local” using the system link option for multiplayer games (Halo, etc)
For the life of me I cannot remember what that was called, and my Google-fu is failing and only returning results for console emulators.
XLink Kai? I remember it from late 2000s, I don’t know much about it now.
https://en.wikipedia.org/wiki/XLink_Kai
https://www.teamxlink.co.uk/
I do believe that’s it! Thanks!
Now to see if it’s still active or even works.