Look in Google for Electret Microphone and you can read all about it.
Its electret material holds a 50V charge to polarize its capacitor diaphram.
Sound produces an AC waveform. An electret microphone has a FET transistor inside that is powered with a low voltage low DC current so its output has a DC bias voltage with the AC sound signal superimposed on it.
A dynamic mic has a moving coil and a magnet like a tiny speaker. Its output is a little lower than an electret mic.
Some electret mics are a little more sensitive than other electret mics.
If the load impedance is less than about 10k ohms then the output level from an electret mic will be reduced.