A: gather moisture from the air B: produce moisture in a way that breaks physics. C: have gaps inside them that Yukari has been using to fill them with water.
I thought that everything they excreted was supposed to be filling in some way? At this point, most artists probably don't care and make them piss normal liquid anyway.
There's at least one cross-section that includes a bladder-like portion, but i prefer the idea that they pee in order to dispose of excess moisture, to prevent melting from the inside.
I basically decided developing pre-birth yukkuri are given a very runny filling to prevent stalk clogging, because I only really see the koyukkuri peeing, can't think of an instance of adult yukkuri doing it, and even if there are it seems MUCH more common for the children.