When there are no sufficient real players the server adds drivatars of real players who had gone through the same race before. What you see are their machine-learned 'ghost' replaying with all their recorded human mistakes, but you can still push and collide with them and the machine-learned 'ghost' will adjust and respond accordingly. Read more: Drivatars "drive like real people, learning from their behaviour and reproducing it in other peoples games. Allowing players all over the world to face off against their friends and other players across the Xbox Network, without ever actually playing the game together."

https://www.gamedeveloper.com/design/how-forza-s-drivatar-actually-works