Air leaves the fan into the plenum, the sealed space under the floor. It passes up through the perforated air floor, through the compost, into the space above, and back to the mixing section where fresh, return and exhaust dampers decide what the fan sends round again.
Every part fails quietly. A blocked floor outlet leaves a dead zone no probe may sit in. A worn door seal pulls untreated air into a pasteurised batch. A choked plenum drain holds water that cools the air from below. Inspect all of them before you fill, because once compost is in you see none of it.