On a not-so-unrelated note, I'm glad Crusader Kings III will be on XBOX PC Game Pass as well
Yes I saw this last night I was like, wow!
Free as long as it's part of the catalog and you probably still have to buy most DLC. This is an educated guess after someone recommended Game Pass to me. Only some DLC is free.
I'll probably try it for free then buy it on Steam for mods.