The IOC set the precident that some male born athletes could compete in female sports but with criteria eg blood tests for testosterone levels.
In amateur sport where blood testing is not usual, this aspect is dropped. The principle that a male athlete who identifies as a female can compete remains.
There cant be discrimination as to whether a transgirl/ transwoman should have surgery, take hormones etc
So to quote Dr Harrop, a woman (and by inference) a girl is someone who identifies as such.
Girls and Women's Sports are no longer based on sex.