none of the above.
i figure out what the monsters have, what they are using, what they have stored away to keep out of the good guys hands, what they are making, what they are hoping to find, etc...
all ahead of time. this is done randomly. but after figuring out all of the above. it ain't random anymore. it has a back story and figures into the plot. and so on.
from the PCs point of view nothing is random.