the general consensus is that personality disorders form from a combination of genetic predisposition, and environmental factors, more specifically childhood trauma, there are links to personality disorders and neglect, emotional abuse, physical abuse, sexual abuse, etc, and having a genetic predisposition just means that because of ones genetic makeup they may be more likely to develop a disorder, and having other things like neurodivergencies such as ASD or ADHD can make it easier for them to develop as well because of an already increased difficulty in ones environment, also if your parents showed specific traits then you might develop those traits and constitute a diagnoses of a personality disorder
personality disorders do not just develop for no reason, while you can have very high genetic predispositions its not possible to be born with one, for an individual cannot just be born with narcissistic personality disorder, those behaviors are learned, there is an exception for antisocial personality disorder, specifically psychopathy, because psychopathy can be caused by specific brain structures that can cause one to be diagnosed with ASPD, but generally people are not born with them