What an ignition coil actually does
An ignition coil is a transformer. Primary windings and an iron core. The secondary steps that collapse up to tens of kilovolts. It is not a spark plug and not a distributor.
The coil is a transformer. A thin wire from the PCM — or an older ignition module — puts battery voltage on the primary. That is not the spark. The spark is what leaves after that field falls down.
Primary windings sit on an iron core. Current builds a field. The box opens the circuit. The field collapses. The secondary — thousands of turns — steps that collapse up to tens of kilovolts. That pulse goes down a boot or a wire to the plug. The plug is the gap. The coil is not the plug. It is not a distributor.
Coil-on-plug sits on the well. One coil, one hole, a boot you pull. A rail module can cover four wells at once. A remote can or a wasted-spark pack lives on the cover or the inner fender and feeds more than one plug through wires. Same transformer. Different walk to the well.
A misfire or a rough idle is often this unit dying, not the plug. Cracked boots, a coil that is hot, a code that follows one cylinder when you swap the can — start here. Do not start at a four-pack of plugs because the scanner said misfire.