Are there different types of mass airflow sensors?
Yes. There are ones that give an analogue output, and ones that give a digital output.
What should the voltage be of a healthy mass airflow sensor?
On a digital sensor the voltage difference between minimum and maximum should be around 4.5 volts. If you get a different voltage reading, then it would indicate an electrical fault in the wiring. As always check your data source as more sensor type with different operating voltages are entering the market.
What should the oscilloscope frequency look like of a faulty sensor?
In a healthy airflow sensor, the signal from the sensor should increase with the RPM of the engine. If the frequency shows a flatline, or a shallow increase that doesn’t match the speed of the RPM change, then that would indicate a fault with the sensor.