Okay thank you very much for clearing that up!

So does that mean that cross-platform play for any game would not work since I do not own an Xbox? Or just that the Xbox app is not essential to the cross-platform play and I do not need an Xbox?

I do not own an Xbox one or an Xbox 360, but my friend has an Xbox 360 and I have a windows 10 gaming PC. I was able to add him as a friend and send messages, but I see that a number of games do not...