Lougle, I would explain it this way, without using a trans woman as an example, because I'm not the best person to explain that.
If there is someone who has been pregnant multiple times and that has been very important to them, they might say that their experiences of pregnancy are what makes them feel like a woman. They feel their other experiences are just human experiences, but this one is what mostly makes them identify as a woman on the basis of biological sex, not gender.
To some other woman, pregnancy may be totally irrelevant (whether she has been pregnant or not). She may feel that what makes her identify as a woman is that she has been socialised as a woman and feels she shares that with other women.
To another woman, this socialisation is totally irrelevant. She may feel that she is innately feminine and likes quilting or floaty clothes or similar. She may feel that there is a thing called the female brain that is feminine, and she has one, and this is why she identifies as a woman.
To another woman, femininity is totally irrelevant. She has a very strong feeling that she should have a body that looks traditionally female (breasts, a vulva). If she had a body with a penis she would feel traumatised. Her identity as a woman is based on her feelings about the external signs of biological sex. She believes in the female brain in the sense that the female brain feels right in body that is externally typically female in appearance.
To another woman, the appearance of her body is irrelevant. She has a very strong feeling that she has an innate sense of being a woman, but she wants to be an engineer and feels a sexist world is putting up barriers. She believes in the female brain but in a more abstract way, not connected to the body or roles.
This is obviously a simplification as many women will feel a combination of these things. None of these women have to be trans women, but they all have different reasons for defining themselves as female. If one of us meets someone and they say they are a woman, we don't have to assume that pregnancy was massively important to them, or that they were socialised heavily as female, or that they are feminine, or that they have a strong feeling about the appearance of their body, or that they have a strong opinion on gender roles. We just don't make assumptions that all women are in some way the same and must agree to that and be that, simply because they called themselves a woman.
And some of these things are incredibly sensitive. Some people don't want to make a statement about gender, or femininity, or biological sex, or their fertility. Others can't avoid making a statement about one of those things because it is so obvious about them, or it would make them utterly miserable to hide it.
There are still structural problems and to resolve them we have to look at particular things and tackle them collectively - lack of pregnancy care, hatred of femininity, gender stereotyping and so on. We have to be able to quantify that X disadvantage happens to women or group of women Z, which requires particular definitions of what a woman is in particular cases. But on an individual level, there is no reason why all women have to feel the same way, or why I must insist that because I do or don't feel something, I am somehow invalidated if every other women doesn't admit she feels it too. And it is impossible to impose one set of feelings on everyone, because all of them are telling the truth about themselves.