According to Jenner he has had "the surgery", if that's what you were referring to. Changing sex isn't possible, but 'transition' is the trans version of coming out (when they refer to their transition as "coming out" instead, that's just appropriation). They transition by changing their name & pronouns. Nowadays they can "legally transition" too. Often they "medically transition" in addition to that. Once society starts treating them as if they were female, whether they pass or not (celebrities like Jenner will never pass because everyone knows the truth) that's when they have "fully transitioned". Transition is often used as a shorthand for medical transition, but we should probably distinguish between the two terms more often, in order to avoid confusion. Full medical transition would include genital amputation/inversion, which 30% get done.

Transgender people believe that people "are" or "have" a gender. They are confused. Gender doesn't work like that. You can't have masculinity, or femininity. They are using gender as a shorthand for gender identity, likewise they are using gender identity incorrectly too, as what they are actually referring to is a "preferred sex".

Transgender person: someone whose sex & preferred sex are mismatched

Transgender is an umbrella term & used to include cross-dressers, transvestites & transsexuals. Nowadays it includes transvestites & transsexuals, but excludes cross-dressers. The movement is androcentric, binary centric & centres trans medicalism. The assumed/default/normative transgender person is a gender dysphoric transsexual man who identifies as a woman & is on hormones & has had a number of cosmetic surgeries to alter his male anatomy to resemble female anatomy to the point where he passes – this demographic is the privileged social majority within the microcosm.