"Are you dumb, have you read giants TOS? "
Have you ever heard the statement, "Better that others think you a fool than to open your mouth and remove all doubt." From your last comment, I believe all doubt has been removed.
You obviously have no understanding of what open source means. If the game was open source, the entirety of its code would be available for users to make changes to. Open Office is one example of this. Most of the code in FS25 is locked and can not be modified or even viewed easily by users. While you can view all of the code for a mod, these fall under copyright. I am assuming you are under the age of 21 based on your comments.