Guinea pigs themselves should not really smell but they are not completely odour free either but with good maintenance you shouldnt smell poop or urine but you will always smell hay - hay is essential to them, they need lots of it all the time, so if the smell of hay is going to be irritating to you, then piggies will not be the right choice for you. Hay, however, should always smell sweet and is a rather nice smell!
Wet hay will smell if it isn’t changed frequently enough. If the fleece doesn’t wick properly or the absorbent layer underneath isn’t doing its job properly, then that can increase the risk of smells, not having a big enough cage can make smells worse, or lack of ventilation. Using pee pads in high traffic areas can help as they can be removed more frequently without having to change all of the bedding.
So you can see that if you do notice any smells, then there is usually a reason for it which can be rectified.
I have two boars (they live in my shed) but being boars they love nothing more than scent marking fresh bedding so sometimes, if they are feeling particularly dominant, then there is a whiff of boy smells in their shed. Sows do scent mark but they tend to not be quite as strong smelling.