Why would any of it show up in the code? As I recall, what Yelp is accused of is making fake bad reviews that go away if you give them money. That could easily be a manual process done by sales people.
if business_has_paid_extortion_money:
show_good_reviews()
else:
show_bad_reviews()
There is also no code to allow manual review ordering. They are sorted according by an algorithm with a variety of inputs including who you are friends with, who has Elite status, etc.