To get started, you'll need to go to "My Games & Apps" followed by "Apps" and then select the "Xbox Game Pass" app. In there, you should see Starfield (or you can search for it). Select the game and a pop-up menu should appear in which you see the option to "Play" with a little cloud icon around it. Choose this, and you'll be playing Starfield on your Xbox One - streamed to your console via Microsoft's servers!
The caveat here is that Xbox Cloud Gaming requires a strong and stable internet connection, otherwise you may suffer visual issues and instances of heavy lag. If your internet is good enough though, it should almost feel like you're playing the native version of Starfield on your Xbox One (without needing to buy a new console for it!).
At the very least, this method allows you to try Starfield for yourself and consider whether it's worth shelling out for a new Xbox Series X|S or PC in order to download the native version in the future. Enjoy!