Brian Ansorge Said:
"Can I download Arial family of fonts? and where?: I somehow deleted the Arial family of fonts (go ahead, call me "stupid") which I believe came with my macOS. Is there somewhere I can download them from?"
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Reinstall the macOS:
Anything that has been lost would be reinstalled (including these fonts, if these particular fonts came installed with your macOS).
1. First, Back up your Mac:
Create a Time Machine backup of your Mac, so that you can have something to restore your Mac from, should anything go wrong with the reinstall.
2. Then, Reinstall the macOS:
Nothing would be deleted. It's just that items that have become corrupted or deleted, would be replaced or reinstalled back to where they should be. Go Here: How to reinstall macOS from macOS Recovery - Apple Support