Thank you for your kind and detailed explanation. I’m worried the question is too simple to be answered.
Referring to your answer, I reread the code twice. I was fundamentally wrong before.
Now my understanding is: 【sequential】 is like a big box, in which 【layers】 are arranged according to the transmission order of neural networks. Then encapsulate the big box. When calling,  only need to pass the actual parameters to 【sequential】, and the program will automatically execute layers according to the sequence. After executing the last layer, it will return the calculation result.