Material and manufacturing
Main reason to use electret microphone is low cost of manufacturing. Material choice is based on usage and low cost.
(PIC)
Electritet microphone consist of the amplifier transistor connected to the pcb. PCB can be made of silicon or ceramic. PCB is just for keep transistor inside and cancel noise coming from both sides.
Electret is a very thin polymer foil.
Aluminium cover is used to cover from dust.
Plastic is used as spacer so the air can go inside.
On top of the electret microphone is glued noise reducing porous material.
Manufacturing electret microphone is simply attaching pieces together. It can be homemade.