Whenever a product is added to cart an additional qty of same product should be added to cart [closed]

Question

Should be achieved by preference not using the plugin and observer

Answer 1

You can create a preference for Magento\Checkout\Controller\Cart\Add::execute

Or you can create a plugin for

\Magento\Checkout\Model\Cart\RequestQuantityProcessor::prepareQuantity

Check if the current request is add in plugin ode

And update the qty as per your logic