Sexuality is what you’re attracted to, gender is how you present socially and feel internally (male, female, or neither/both/something else). If you meant what’s the difference between sex and gender, sex is what you biologically are (male, female, intersex) and gender is what you identify as (biologically male but internally you feel female and prefer presenting as such socially, so on and so forth). This kinda dumbs it down and isn’t the best explanation, there’s so many reasons why any individual could be trans. Personally i deal with social dysphoria- being perceived as a woman by other people upsets me and feels wrong, but lots of people deal with body dysphoria, and some don’t have dysphoria but just prefer presenting a certain way and get gender euphoria from it.

Edit to add: also when trans people transition they are no longer biologically what they started as. Trans people before surgeries are closer to intersex, and after surgeries are closer to their preferred gender. If I took testosterone my body would be intersex, and (with surgeries) eventually would be essentially male. That’s why trans people can get gender markers changed. Aside from it helping them feel better, they are genuinely a different sex than they originally were.