Refer to figure.
A magneto is a self-contained electromagnetic generator specifically designed to produce the high-voltage electrical pulses required to fire the spark plugs of a piston engine. The magneto generates its own electrical energy entirely independently of the aircraft battery, alternator, or any other external electrical source.

The magneto contains a permanent magnet mounted on a rotating shaft that is driven mechanically by the engine accessory gearbox. As this magnet rotates, it creates a continuously changing magnetic flux through a soft iron core wound with a primary coil of relatively few turns. By Faraday’s law of electromagnetic induction, this changing flux induces a low-voltage alternating current in the primary circuit. At the precise moment of required ignition, a set of contact breaker points interrupts the primary circuit. The sudden interruption causes the magnetic field in the core to collapse very rapidly. This rapid collapse induces a very high voltage, typically 15 000 to 20 000 volts, in the secondary coil through transformer action, because the secondary coil has a very large number of turns compared to the primary. The distributor then routes this high-voltage pulse to the correct spark plug at the correct moment. 

The entire energy cycle originates exclusively from the kinetic energy of the rotating permanent magnet, which is converted to electrical energy through electromagnetic induction. 

Boosting power from the battery → INCORRECT. The magneto requires no power from the aircraft battery to generate high voltage or to fire the spark plugs during normal engine operation. The battery may be used to power a booster coil during starting on some aircraft to supplement the magneto output at low cranking speeds, but the battery plays no role in normal magneto operation once the engine is running. The magneto is entirely self-generating.

A booster coil → INCORRECT. A booster coil is an auxiliary starting device used on some aircraft to supplement the magneto output when engine cranking speed is too low for the magneto to generate sufficient voltage on its own. It is a starting aid only and does not contribute to high voltage generation during normal engine operation. The booster coil is powered by the battery and is external to the magneto. It is not the source of high voltage in normal magneto function.

Boosting power from the generator → INCORRECT. The aircraft generator or alternator produces electrical power for the aircraft electrical bus, battery charging, and electrical loads. It has no connection to the magneto ignition circuit. The magneto does not draw power from the generator under any operating condition. The two systems (magneto ignition and aircraft electrical system) are deliberately kept independent of each other for safety redundancy.

A rotating magnet → CORRECT. The rotating permanent magnet is the sole and fundamental source of all electrical energy in the magneto. As the magnet rotates driven by the engine, its changing magnetic flux through the primary coil induces a primary current by electromagnetic induction. When the contact breaker points interrupt this primary current, the rapidly collapsing magnetic field induces a high voltage in the secondary coil through transformer action. All energy originates from the rotation of the permanent magnet, which is itself driven by the running engine. No external electrical source of any kind is required.