>>509867693 (OP)In about 1% of fetuses (yes, it's theorized to possibly be that high) an additional X or Y chromosome is passed on from the parent to the baby. It's actually more likely if one of your parents is intersex. It's actually not that hard to understand and it almost always has zero physical symptoms. Intersex people can actually look completely normal and have no idea at all that they're intersex.
In some cases you also get stuff like androgen insensitivity syndrome (AIS). People with AIS have XY chromosomes, but during development the fetus can't use the androgen the mother's body produces. So they wind up developing the female body type despite being genetically male. They're usually infertile, but because their bodies literally can't produce or use androgen (the stuff that makes you masculine, not testosterone contrary to poplar belief) they actually wind up looking EXTREMELY female. Like more female than typical XX women who's bodies do produce small amounts of androgen. Some people will argue that people with AIS are perfect women despite being genetically men, as a result. And because androgen has a huge deal to due with physical looks as people age, AIS people tend to look young for longer. They effectively become traps who don't experience a wall.