This is a perennial subject of debate that can never be answered in any objective way. It is clear that both biology and experience play some role in a person’s personality. Different researchers believe that one or the other of these factors is the more powerful.
It is clear that people are born with certain characteristics. Parents know that even as infants their children do not all act the same. This definitely gives credence to the idea that there is a biological aspect to our personalities. The same is true of the fact that personalities do not tend to change greatly over time.
However, this does not mean that only biology matters. People may be born with certain tendencies and characteristics, but those attributes can be softened or emphasized by their environment as they grow up. For example, a person who tends to be short-tempered may have that tendency sharpened by parents and friends who get angry easily. This process of socialization will tend to indicate that the person should not try to suppress their temper. By contrast, the same person brought up in a different situation might have their temper blunted by experience.
Clearly, then, both biology and experience play a role in determining our personalities. Different researchers come to different conclusions as to the relative importance of each factor.