Compost leaving Phase I carries roughly 800 to 1,000 ppm of ammonia and sits at pH 8.2 to 8.5. That ammonia is nitrogen, and it is not thrown away. During conditioning the thermophilic community takes it up and locks it into microbial bodies, which the mushroom later digests. Some also leaves with the exhaust air.
Do not call this one single pathway. Uptake, chemistry and gas loss act together, in shares set by air supply, temperature and the nitrogen the formulation put in. A mix stacked above 1.75 percent nitrogen leaves more ammonia for these days to clear.