They go when they need too . Water bowls , along the edges of there enclosure in the corners . And you can tell when they are about to go , they will look quite swollen near there vent .
I guess that’s a question that’s hard to prove.
However, anecdotally, I have two youngsters that always do it in the water bowl every time and never in the hide or else ware. Why give predators a heads up about where you are hiding (through sent) if you don’t have too? Many mammals even eat their babies poo to hide the scent from predators, and even eat the placenta for the same reason. Its not evolutionally and survivaly advantages to do otherwize.
I have heard ball pythons move hides when they smell to much of them in the wild.
Those two ball pythons of mine that only use the water bowl cant actually get in the water bowl as the bowl is too small to submerge more than the tail in, so I dont think its the water soaking helping them to go.
(I usually have bowls snakes can fully get into, but currently not for these younglings )
I would suggest they have some control and at times none, like anything.
Never seen any snake poop while eating either.