How do I fix sink sweating under bathroom cabinet, causing mildew.

Put a thermometer in the vanity cabinet with the drawers and cabinets closed to see what the temp is running compared to the rest of the house. A BBQ thermometer with remote would work well for this. You could also instant read thermometer and check the temps of the drywall. You could have poor insulation in that outside wall and also hot air coming in around the pipes.

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I get the issue

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Talked to delta and plumber said issue was from the parts inside the the handles of faucets.

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Is it possible to have humidity in these cabinets caused by the materials the sinks and granite are made of?

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I'm not sure what is the issue but a thermometer or some temp measurements could rule in or out if you have hot air coming into the space from the outside wall. If you do have hot air coming in from the outside wall. You would need to insulate the wall cavity or spray some expanding foam around the holes where the pipes go through the drywall.

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plumber said issue was from the parts inside the the handles of faucets

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This would mean a bad valve stem. But that doesn't cause "moisture" so to say; it would be a leak and you would notice it.

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two sinks are along the outside walls of the home.

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no issues are seen until this summer

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These two things stand out to me.

Assuming you live in the south, obviously we have terrible humidity. And believe it or not, there is hot humid air inside of exterior wall cavities since its not a conditioned space.
Hot air can hold more moisture than cold air. When hot humid air meets cold, it condensates. (like a glass of ice water condensating on the outside)

I say that because I am wondering if you maybe have hot humid air coming from the inside of the wall cavity, thru where the water/drain lines penetrate thru the finished wall, under the sinks, and possibly causing a slight condensate promoting mildew/mold growth.

While I've never of heard of this in particular happening, in theory it makes sense and sort of the only thing that makes sense given what you've said.
Of course, this is all assuming there's no leak.

ETA:
RaginRampage, so I am not crazy for thinking this

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Are there any battery powered dehumidifiers that are sold? Don’t have electrical socket under sink.

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these don't require power. they have hygroscopic chemicals in them that attract the moisture from the air.

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I will put expanding foam insulation to fill the little voids when I get home.

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that should fix it, honestly. there's no air movement in that cabinet so that air is stagnant, or like someone pointed out, the ceiling vent may be sucking hot outside air in.

This fixed the problem! No more moisture under the sink. Thanks!!