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The starter has a solenoid (relay) on it. The ignition switch energize the solenoid, which pulls a fork to engage the starter gear with the crank shaft. It also closes contacts for the starter motor, which draws too much current for the ignition switch to carry. Most likely these contacts are burnt and/or covered in the black soot that you say covers the rest of the starter.
Measuring resistances will not show you where the problem is. Suppose the starter motor draws 100A.That means the motor has a resistance of 12 / 100 = 0.12 Ohm. If you cabling also has a resistance of 0.12 Ohm, this means the motor will see only 6V instead of 12V. To measure less than 1 Ohm, you need to use a 4 wire measurement to correct for offsets in the test leads, a function which is available only on the most expensive test meters.
The correct way of jump starting a car is by connecting the negative lead from the good battery negative terminal, to the engine of the dead battery. This is so you bypass the ground straps which may be bad, and it also puts some resistance between the good battery and the bad battery so that the bad battery does not draw all the juice from the good one. Most people connect streight from battery to battery, but this is not the correct way. It delivers more power to charging the dead battery than to start the engine. The same goes for the positive lead, but it's not so easy to reach the positive terminal on the starter motor.
Have fun...
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