I noticed that the Fiesta '23 is in the game and that people are driving it. I see that it is available in the winter seasons but I have no idea what to do or if I can even get the car already. Any help is much appreciated!
The first car to come out yesterday was the Focus ST; that’s probably what you noticed.
I saw one in my trial that I initially thought was a “hacker,” but it turned out the AI was using it! 
You can’t get it legitimately yet; you’ll have to wait until winter.
You might have seen the Focus painted green. If not, there are two possible explanations: either someone hacked into the game, or it was chosen as a reward from ongoing competitions on the forum. You can select a car that’s already in the game, and the Fiesta has been available since last week’s update. You can set it to appear in blueprint events, but I’m not sure if there have been any recent contests.