You're thinking in the right direction, but you're not quite there. Yes, gender gets assigned at birth as your biological phenotype (because the overwhelming majority of humans identify as cis gender). Humans become individuals and eventually realize that maybe their assigned gender does not fit them. At this point it becomes their own choice and it overrides any gender assignment given at birth. Depending on the individual, gender changes from something that's assigned to something you assign yourself.
You're thinking in the right direction, but you're not quite there. Yes, gender gets assigned at birth as your biological phenotype (because the overwhelming majority of humans identify as cis gender). Humans become individuals and eventually realize that maybe their assigned gender does not fit them. At this point it becomes their own choice and it overrides any gender assignment given at birth. Depending on the individual, gender changes from something that's assigned to something you assign yourself.