Generally, there will be a huge difference between how a game looks when you're playing versus what it's like in trailers.
Mind you, this isn't usually the result of an attempt to mislead fans. Video game developers strive to craft the perfect trailer for their game, wanting it to showcase the game in the best way possible. Forza Horizon 5 is no exception. As awesome as it looked at E3 2021, the trailer is different from actual gameplay footage.
After all, Forza Horizon 5 isn't a next-gen exclusive. It's releasing on both the Xbox One and Xbox Series S/X consoles. This suggests that Playground Games will need to make compromises, so that the game can run well on the older generation consoles too.
With that said, whatever concessions Playground Games had to make, they're not visible to the naked eye.