Metabolic water is the starting point. Oxidising carbohydrate yields water as a product, and a seed diet supplies enough that a kangaroo rat needs no drinking water at all. The difficulty is holding on to it.
The kidney does much of the work. Kangaroo rat kidneys have a long loop of Henle and a relatively enormous renal medulla, and can concentrate urine to several thousand milliosmoles per kilogram, far beyond human capacity. Faeces are excreted dry. Respiratory loss is countered by countercurrent heat exchange in the nasal passages: exhaled air is cooled on the way out, condensing water back onto the nasal tissue rather than losing it to the desert.
Behaviour completes the budget. The animal is nocturnal and spends the day in a plugged burrow where humidity is far higher than at the surface, and it stores seeds there, where they absorb moisture from the burrow air before being eaten. It also does not sweat or pant.