What determines the kind of body one gets in the next life? Is it just random?

The process is not random according to the principles outlined in Chapter 8. The text explicitly states that the kind of body one takes is determined by "the knowledge earned and as the performance of deeds." This is presented as a principle of the Vedas. The compassionate Mother Shruti (Vedas) is quoted as saying, "Birth happens according to one's wisdom." This means the soul's accumulated knowledge and past actions, or karma, dictate its future embodiment. For example, the text mentions that many sinners enter the womb to take a body through their karma, while others who have been in the animal kingdom may even return to a stationary state like plants or stones based on their deeds.