Is an employee obligated to send design prototypes that were neither sold nor implemented to the former client for whom he worked, or is he entitled to keep them? And if he is obligated to send them, does he send all of them?

You must speak truthfully and give the employer everything related to his work, including the remaining five designs, unless he permits you to keep them. This is because they belong to the work, not to you. You are entrusted with what was placed in your hands, and concealing them is a betrayal of trust. Allah Almighty commanded that trusts be rendered to their rightful owners, and the Prophet, peace and blessings be upon him, made betrayal of trust one of the signs of hypocrisy.