Here is the diagram and adapted explanation that is now appropriate for explaining why the launched cart decreases in speed during the collision!

Explanations Using Energy Ideas – Adapting an Explanation

(1) The launched cart is moving toward the stationary target cart and when they collide there is a contact push/pull interaction between them. (2) The energy giver during this interaction is the launched cart, and energy is transferred from it to the target cart, which is the energy receiver. (3) Because of this energy transfer, the kinetic energy of the launched cart decreases while the kinetic energy of the target cart increases. (4) Since the launched cart was moving with a certain speed and had some kinetic kinetic energy, this decrease means it will have less kinetic energy, which in turn means that its speed will have decreased.