They rate USA 75% for corruption, and the (ex) British colonies as 100% for corruption. What does that mean? Is literally every single transaction in those places corrupt? "Out of 4" seems like a very poor way of doing this kind or rating.

Higher score is better. 4/4 is best.

Compare vs. China https://freedomhouse.org/country/china/freedom-world/2020

You can't just turn ratings out of 4 or 5 or 10 into percents like that. It doesn't mean the same thing. So no they don't rate those countries that.