It's read noise. The same amount of read noise is present in the entire photo, but in the brighter areas there's enough light to overwhelm it and you can't see it.
How to I fix this?
Shoot in brighter light and/or expose appropriately for the shadows.
If there's too wide a range between the brightest areas of the scene and the darkest so that the ...
You can define a preset manual white balance in the camera - Nikon D3500 manual, page 116
Assuming that fixes the white balance issue, you're still going to have to run these through Photoshop anyway.
You need to straighten, crop, remove the dust & scratches from the watch glass [unless you're selling them warts & all] & probably sharpen up a ...