How is the Xbox market related to SteamDeck? SteamDeck plays PC games (which XBox doesn't), and XBox plays its own console games (which PCs and thus the SteamDeck don't)

The Venn diagram of Xbox games and PC games is very close to the Xbox circle just being completely inside the larger PC circle. The number of Series S/X exclusives is literally zero at this point, at least I'm fairly sure, and if there are any, my bets are on them being shovelware. The Xbox One only has one exclusive game of note, which is Halo 5.

I don't think the performance of the Steam Deck is up to play all Xbox games at Series X quality, but that's a nitpick assuming future Steam Decks arrive.