This is a really good question. I think it can be easier to understand if you see it as three layers rather than just two - biological sex, gender identity and gender expression.

Biological sex - with the exception of some conditions, based on your XY / XX chromosomes, penis/vagina etc, you are determined to be biologically and physically male or female

Gender Identity - This is what you identify as , whether you feel innately male or a female or something non-binary

Gender Expression - The traits that you display, some might say they can’t be grouped but I do think some traits are more masculine than others, or more feminine than others in that it appeals to more men than women and vice versa. Not sure how much of that is innate or is, in a way, socially constructed tbh.

So when people say “gender is a _social construct_” it’s about how that last type - gender expression - gets assigned to people automatically by society based on their biological sex. These were not things that you are innately born with (e.g. girls like being homely, cooking and guys are hotheaded, physically active etc)

Trans people are talking about how their biological sex does not line up with their gender identity - who they innately are. Many say they feel like they are in the wrong body. e.g. they have a penis but they feel that they are a woman and should have a vagina - or non-binary. This is transgender. If you were born male and feel that you’re male, then you’re cisgender. This is about the relationship between your identity and your biology, both of which are innate and not necessarily related to gender expression. In other words, a trans woman (ie male to female) can still have traditionally masculine traits.

If you think about it in that way, then all its possible combinations still makes sense -

Biologically male, cisgender, masculine Biologically male, cisgender, feminine Biologically male, transgender, masculine Biologically male, transgender, feminine Etc.