How does a soul pass into another body?

Lalwani explains the 13th verse of chapter 2 of Bhagavad Gita for happiness. He says that the soul goes to another body after death. This is a continuous process. However, those who do good deeds are saved from this process. They are given a spiritual body in the next birth and finally, mortality. Krishna tells Arjuna not to grieve for these elders because they are going towards salvation.