As a cis woman, I don't know that there was ever a particular time when I "knew in my core" that I was a girl. I just knew I was me, and I was told by my parents and society that being me, with my body parts (different to my brothers who my parents and society told me were "boys") was the same as being "a girl". Thus I created the association between the subjective experience of "being me" and the definition of "girlhood" that society created. But did I know inherently? If I was born on a deserted island and no one told me if I was a boy or a girl, would I know? No I don't think so.

Is my experience the same as all cis girls? Almost certainly not.

My partner is nonbinary and his experience with gender seems to be tied to his feeling of being like a "shapeshifter" internally, that if he could shapeshift into different physical forms depending on mood, he would. And some of those forms would be female.

It's all very interesting, I am always very curious about what leads someone to their gender identity.