You don’t describe your setup at all! How do you think anyone understands what you are doing, how you want to achieve it or what you are expecting?
Are the cloths also skinned, are you sure you are using the same bones for them as for the character. What is the morphing used for?
The Problem, picture explains. There is nothing incomprehensible. I’m using character “BlendShapes” for the body. But, as seen in the picture. There is a problem. How outfit recedes?
The T-Shirt either needs blend shapes, or you need to find a way that it fits automatically. One possibility would be to write a script for that. But that is probably not such a simple task.
There is a system on the Asset Store that will do what you need but it uses its own Blendshape system and not the new Unity one. It will automatically move any clothes to match the deforming base character, so its doable but I think it would be quite a challenge to write it yourself.
Sorry I don’t understand what you mean ‘Must such a code’ If you arn’t prepared to pay for a solution to your problem then you will have to take the alternative step of investing your time to figure out how to solve and code a solution yourself, which I suspect would take a couple of weeks depending on your coding ability.