Airplane compasses are indeed affected by the electrical system, but not so much as to make them unusable. A maintenance process known colloquially as "swinging

Airplane compasses are indeed affected by the electrical system, but not so much as to make them unusable. A maintenance process known colloquially as "swinging the compass" involves turning on the electronics, positioning the aircraft over a compass rose painted on the ground, and noting the error at various bearings. As for the metal, most metal airframes are aluminum.