Wherever they are when the urge strikes (nude) (incontinence) (poop)

... Pretty much anywhere, as if they were babies with no diaper on.

There will be no toilets, so it will be normal to just let go automatically, while doing anything really. You will also be expected to poop wherever you are in much the same fashion (although you will be expected to pick up after yourself in the same way you would do for a dog). If you hesitate to pee or poop, you will be perceived as having a medical problem.

You will also find you have no bottoms. This is because in this world, people don't wear them, or else they will get messy. In fact they will typically go around nude unless the weather or some other condition calls for it. If you must wear bottoms for whatever reason, they will be waterproof and easy clean in case you get human waste on them. And you'll wear a diaper underneath. Given that people don't wear anything on their lower half, It may not surprise you that wearing a top is optional, and is only done to keep warm"