We don't seem to have a shared understanding of what gender is.
And therein lies the rub.
While there is no consistent definition of gender then it’s easy to end up talking at crossed purposes.
My belief is that women’s oppression is rooted in our sex and therefore our biology.
At a purely biological level our XX chromosomes mean that as an embryo and then a foetus we develop ovaries, uterus etc. During and after puberty the predominant oestrogenisation of our bodies mean that (generally) we develop physical characteristics designed for gestating and nursing young. Pubertal changes in Males on the other hand are designed around being able to hunt and find food, defend themselves (and potentially their female mate and young) from other males and to be able to find other females to mate with in order to ensure survival of the species. Early humans did this in the same way, broadly speaking, all mammals do.
In humans however, evolution means we have developed a complex series of social rules that have changed over time but were initially rooted in our biological reproductive functions. So again, even as we became hunter gatherers, farmers who domesticated animals etc the roles in society (what we could call gender) were linked to the our reproductive functions.
As time has gone on, more complex social rules are added that say ‘men should be strong’ ‘boys don’t cry’ ‘girls are delicate and aren’t as clever as boys, ‘women’s brains aren’t able to do science,’ ‘women are too emotional to hold senior positions in society.’ These come from societal ideas that originally started with fallacious ideas about human biology that we now know is untrue, just as many things we believed about human biology we now know to be false. These are societal expectations and (I think) are at least what some feminists mean when they talk about gender.
Of course, the fact that we exist within society and within these expectations (such as telling boys not to cry or girls they can’t be doctors) means that some of our behaviours are the product of our internalisation of societal norms. Boys may be more aggressive in part due to inherently higher levels of testosterone but are influenced heavily by societal expectations that they are not supposed to cry or show physical weakness as to do so is to be considered less male.
Feminists like me (what might be broadly considered ‘radical’ feminism) argue that our oppression is rooted in biology (the fact that only we can gestate and nurse babies and the inherent vulnerabilities and changes to life’s structure that brings with it) but that the other behaviours expected of us by society are not inherent to our sex and therefore should be ignored.
TLDR: Human behaviour is rooted in our evolution but subject to the prevailing attitudes of what a society dictates those sexes should do (gender). Some feminists argue that the important, inescapable features are the biological ones and the rest (gender) should be abandoned.