It's the inside of the wheel, don't stress about it.
Looks like that because they never had any protective finish from when they left the factory because no-one looks there except mechanics.

To get that to shine, you need to spin it up and rub it with 300, then 600, then 1200 then buff em.
Buffing alone would take an eternity to remove the oxide layer.
But I wouldn't bother unless it was a show car.

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