They do heat up, but I think of the heat as a by-product of the process. I seriously doubt that the sensor gets anywhere near hot enough to ignite anything. Wouldn't that be a nice bit to have, the sensor sending little flaming balls of filter debris straight through the manifold to a bunch of injectors burping up gas?
The change of resistance in the wire is what determines the mass of the air flow (tricky, eh?
The voltage signal returned to the computer will increase as more cooling air flowing across the wire reduces the resistance. Five volts in and .7 returning generally indicates idling flow at a reasonable air temp and baro pressure. Drop the temp and more volts, Increase the baro pressure and the same occurs. When using a scanner, many newer cars show both voltage and grams/second readings, the latter obtained from a look-up table in the computer.