Geneticist here! Essentially the Y chromosome is only important for sex determination because it contains a gene known as SRY, which codes a protein responsible for turning on numerous other genes that are responsible for male sex determination. So what can happen is that the Y chromosome can have a mutant SRY gene or lose it completely, resulting in issues with sex determination or with fully female individuals while retain a Y chromosome.

This is an intersex person, having been born with female gametes only, but having the sexual chromosomal makeup of a man.

Basically, the idea of XX vs XY for male and female is greatly oversimplified. What really matters is the actual genes on that Y chromosome and the genes that the control as well.

https://www.ncbi.nlm.nih.gov/books/NBK9967/#:~:text=Primary%20sex%20determination%20is%20the,at%20least%20one%20X%20chromosome. According to this literature the defining characteristic of biological sex is chromosomes

Out of every 1000 people, 17 are born intersex

Because she isnt transgender. She is a BIOLOGICAL WOMAN.

Massive simplification: The “default” development of a fetus is female until genes on the Y chromosome kick in to make a baby boy. If something interrupts that Y chromosome or the baby’s body doesn’t react to those hormones- you get a girl. The particulars vary from condition to condition. Some may have very high testosterone and not be able to utilize it. Some may have parts that develop differently. Depends on the condition.

It's called swyers syndrome

Bc they are intersex.

Yes. So basically any doctor can solve this riddle for you.