This shows the simplified starter schematic.

The starter relay is triggered by the key and ignition lock, when active the 12v of starter relay terminal 87 feeds power to the starter

terminal 50 which activates a relay in the starter itself spinning the start motor.

There are two wires on the largest  starter post for power up to 1986 model. One goes to the battery, the other to the alternator.

And there is a small wire on the smaller post which operates the internal relay and its energized by using the ignition key.

Starting the 1987 model there is an additional wire on the largest post (so there are 3) which connects power via the wire loom WL2 to the ABS module pin #12 (main power to run the ABS pump)