How is Leia a princess?
When a prince and a woman love each other very much, they get “married” and the woman becomes a princess.
Actually, Leia is a princess because her adoptive mother was a queen.
It is not explained in the films. But as has been said, Warb, just because something is left unexplained does not mean it is unexplainable. Before you call out TLJ for not explaining things you wish it had, I suggest you read the novel because it might answer those questions.