Xbox (Beta) app and Xbox Companion App

Just looking on some clarity, I do not own a Xbox console but do have Game Pass for PC, my friends have Xbox consoles

I have both apps installed on my PC (Xbox and Companion App)

1. Do I need them both or does the new Xbox app replace the old companion app?

2. Does party chat work across PC and console ( was trying to connect with a buddy but it was not working ) we were playing Modern Warfare (my on Battle.net him on the Xbox 1 X) I feel like we were able to connect on other games no problem but the example
I remember was Sea of Thieves , does this matter does party chat only work if I am also playing a MS Game or was that coincidence ?

3. Not as important but more caught my curiosity how its possible I was playing a few games (The Sims 4 and Arkum Knight) one was through Origins launcher and the other through Epic Game Launcher so nothing to do with Xbox or my Xbox Gamer Tag, my friend
noticed on the Xbox dashboard I was playing them ( he asked me a random question about those games which is how I found out those games show up as activity) how was this even possible in theory? neither game is part of my Xbox or MS store library ? not that
it matters and I am sure I could turn that off somewhere but still made me curious how its all connected ?

thanks

:)