Should I apply a quick detailer or spray sealant to wet or dry paint?

It depends on the product, so check the label. Many modern quick detailers and spray sealants are designed to be applied to wet paint straight after the final rinse and wiped off while drying, which saves time and adds lubrication. Others are made for dry paint and can streak if water dilutes them. In both cases the paint must be clean, cool and out of direct sun. Use a clean microfiber and buff off any excess so the product does not leave streaks or high spots.