Forza Horizon 5 will support cross-play and cross-save across Xbox Series X|S, Xbox One, Windows 10 PC and Xbox Cloud Gaming.
Steam will have support for cross-play, but cross-save with Xbox is not supported.
Your save file which includes progress and in game items on Steam is completely unique and separate from any pre-existing save file that’s associated with your Xbox Live gamertag. This includes user generated content like liveries. layer groups, etc...