In this case, the measurement is taken using the bypass principle.
A laminar flow element in the main channel generates a slight pressure drop, which drives a small proportion of the full flow through the actual sensor tube. Two heating resistors are wound on thin stainless steel tubes and are interconnected to form a measurement bridge. When the medium passes through the tube, heat is transported in the direction of the flow and this misaligns the previously balanced bridge.
Sketch of a bypass sensor block