This is a flywheel. The flywheel is the very first part of the drivetrain and bolts to the transmission side of the crank. The above picture is a 7 bolt flywheel. 4G63 engines are referred to as either a 6 bolt engine or a 7 bolt engine. The amount of flywheel bolts is what determines whether the 4G63 is a 6 bolt or 7 bolt engine.
The flywheel is also used in coloration with the starter. When you turn the key to start your car, the starter kicks out and spins the flywheel via the “starter ring”. The starter ring is what you see in the above picture around the entire circumference of the flywheel with the teeth.
The clutch disk makes contact with the friction disk of the flywheel. The flywheel acts similar to a brake rotor. When you push the clutch in, the clutch fork pushes the throw out bearing against the center of the pressure plate which releases the clamping pressure. The outer part of the pressure plate, which has a large friction surface, then no longer clamps the flywheel friction plate, so the transmission of power is interrupted and gears can be changed.
The flywheel can be upgraded with various different types of flywheels. Aftermarket flywheels are usually aluminum and weigh significantly less than the stock flywheel. A lighter flywheel means you will have less rotating mass that the engine has to constantly spin.