Fungal infection on both inner thighs. Itchy w/ dark color. Week ago skin clear, started using diaper cream 20% zinc. Caused by zinc or alcohol use?

Tinea cruris. It is very unlikely that zinc containing diaper rash cream or alcohol discolored your skin, and you will need an antifungal cream. See your primary care doctor.
The issue. Fungal infections can darken the skin as can repeated scratching/rubbing. Clotrimazole of tolnaftate (OTC) would be better treatment if it is a fungus. Clearing with a skin barrier like zinc oxide cream suggests it is not fungal. If it persists you may need to have it examined.