Quick answer… It won’t! Composting takes time and, unless you use the toilet infrequently, it’ll fill up before it can compost. The exception may be with a ‘long-drop’ toilet which has a larger container beneath the toilet hut/cabin. Most ‘compost toilets’ are actually ‘compost collecting toilets’, meaning you collect the solids and then add to an external larger container to actually do the composting…called ‘secondary compost containers’.