The process
Cutting and programming, step by step
Making a working car key is two distinct jobs done in sequence. First the mechanical blade has to be cut so it operates the door lock and, on vehicles that still have one, the ignition. This is done either by decoding the existing lock, by copying a working key, or from a key code tied to the VIN. On a vehicle with a mechanical or transponder key the blade is cut on a machine; on a smart-key vehicle the hidden mechanical blade is cut the same way, even though it is rarely used day to day.
Second, any electronics in the key are programmed to the vehicle. The transponder chip has to be registered with the immobilizer so the engine will run, and a remote-head key or smart fob also has its lock, unlock, and trunk functions paired. This is done through the vehicle's diagnostic port with equipment that talks to the car's security module. When a working key already exists, the new key is usually added alongside it quickly. When all keys are lost, the module has to be accessed in a recovery mode first, and some manufacturers impose a timed security wait before a key can be added.
Every finished key is tested the same way: it should unlock and lock the doors, start the engine, and — for a smart key — be detected from the driver's seat without the old key present. If the vehicle turns out to need a part that is not on hand, or a procedure that cannot be done outside a dealer, that is identified during the assessment rather than after the work starts.
One point worth understanding about all-keys-lost work: on some vehicles, adding a new key means the system has to be reset in a way that invalidates any key that is still out there but not present. If you have a key that is lost rather than destroyed — dropped somewhere, left with someone — say so, because it changes whether that key will still work afterward. For most people that is the desired outcome, since a lost key no longer opening the car is the same benefit a rekey gives a house. But it is better to decide that on purpose than to discover it later.