The standard size and type of a water supply line for a kitchen faucet in the United States is usually 3/8-inch compression fitting. It is not 3/8 NPT (National Pipe Thread), as you correctly observed.
Compression fittings are commonly used for connecting water supply lines to fixtures like faucets and toilets. They are designed to create a watertight seal by compressing a ferrule (a small metal or plastic ring) onto the pipe when the nut is tightened. This type of fitting is often used with flexible supply lines, which are easier to work with in tight spaces compared to rigid pipes.
If you are having trouble finding the right replacement supply line or adapter for your kitchen faucet, consider bringing the old supply line to a hardware store or plumbing supply shop. They can help you identify the correct size and type and provide the necessary fittings or adapters to ensure a proper connection. Additionally, you may want to consult the manufacturer's documentation or contact their customer support for specific information regarding your kitchen faucet's water supply line requirements.