What is the essence of being a woman?

In essence, it is the woman who becomes pregnant and the man who impregnates. Other than that, there are many other details about being a woman, but only that one is essential.
In terms of relationships, the essence of being a woman is that you have the biological capacity to become pregnant and give birth to children. That is primarily what distinguishes women from men, who do not become pregnant, although they have the biological capacity to impregnate. Relationships also include many other things than reproduction. They include having dinner together, and ten thousand other things. But those things work pretty much the same way for both men and women.