Major parts of a metal detector usually comprise of main three parts. One is a control unit. It is basically the main input device. Because you can manipulate settings from there. It can be analog or digital. Analog control units are easier to use. Digital control units hold more functions and capabilities. Thus it is choice of the user whichever he or she prefers.
Second part is a coil at the end of the device. A coil is the major output hardware component. It throws electromagnetic rays in the grounds. Consequently the detector performs its primary function. Which is detecting metals underneath the ground. Coils usually come in various shapes and sizes. A coil can be mono, DD, concentric, elliptical and so much more. A coil has most of the contact with the surface. So a coil is preferred if waterproof.
Third part is a shaft. It connects control unit with the coil. It is meant for support. Shafts are often retractable. So the size could be adjusted according to user. The metal detector also has an arm rest. It is above control unit, mostly embedded with another rod.