How does the roof fan move air through the cabinet?

The roof fan works as an extract fan.

It removes warmer air from the upper part of the cabinet, and replacement air is drawn in through the other ventilation openings.

The equipment inside may also have fans moving air in particular directions, so the final airflow pattern depends on the populated cabinet as well as the roof fan.

The fan contributes to the overall airflow system; it isn’t a small square device with authority to overrule every blocked vent and tangled cable beneath it.