If you ask most experts in the field what causes someone to become transgender (as in experiencing gender dysphoria or being gender incongruent), they will tell you that this most likely caused by a combination of biological, cultural, social and personal factors. Most of them will agree that there is a significant biological component.
This is important, because it means that it is hard to dismiss gender incongruence as something imaginary caused by political and cultural pressure from sinister people promoting some kind of "gender ideology". All serious scientists in the field acknowledge that the identities of trans people are real and tangible and the great majority believe there are biological factors influencing the development of gender incongruence.
That does not mean that our concepts of gender are not influenced by culture. They most certainly are, as seen in the anti-trans insistence on gender being reduced to "biological sex", another concept that is as vague and ambiguous as the term "gender".
"Biological sex", as it is used by anti-trans activist, is a political and cultural construct that goes against everything we know about gender variance and intersex conditions, both in humans and in other animals.
In intersex people chromosomes, gonads, genitalia and hormones do not match in wide variety of combinations, and in trans people a persistent and intense experience of a gendered self does not mach the gender assigned at birth. Dismissing this as "illusions" or "irrelevant variations" does not cut it.
What science says about the biological component
Research indicates significant biological factors influencing gender identity and dysphoria, particularly in brain structure and function.