It uses your current rating, average rating of your opponents in this tournament, and your score to calculate your new rating. There are formulae and there are some modifications introduced by adopters. K-factor was also introduced in last few years to account for more and more young talents.
Generally USCF rating is about 100 points higher than FIDE rating, if a person has both.
See https://en.wikipedia.org/wiki/Chess_rating_system.