I have an Xbox One S, so I can play with the new Xbox
You mean online as an Xbox One user against / with Xbox Series X users? Certainly. I don't know which games yet, but Halo Infinite, Fortnite or Minecraft could be one of them.
But if you think you can continue playing your current Xbox One games on Series X, the answer is YES.
Thanks, but I mean, if my friend buys an Xbox Series now and I play on the Xbox One S, we could still play together in a party / games
I guess this will support some games, if not your friend can buy a backward compatible Xbox 1 game and you can play together. It will even be possible for 3 generations of consoles to play together (e.g. Black Ops 2).
I think so. For sure. The Xbox Live System and all software will be taken over. At least that's what the reports said. I'm confident for that.
Thank you