Assuming race, As a white person you are going to struggle to get others to see you as black. Cause those are social constructs. YOU do not get to decide what others see you as when it comes to a social construct. Because the very point of a social construct is that it’s the general social frame work used to see others as.

Ethnicity on the other hand, as a white skin tone person if you grew up in Africa in a tribe of indigenous people then you would be an indigenous person. Ethnicity only cares about the facts.

To mirror that to transgender.

Gender is a social construct, it’s what others see you as and how you categorize into a given communities framework. This differs between communities. For example what defines a male gender in ancient Rome is different then ancient Scotland. Both having male genders that quite literally just do not exist in one or the other.

While sex is the physical only caring about the factual biological. The actual flesh and reproductive organs. You either have them, or you don’t. Primary or secondary. You can alter them with modern medicine sure, but even after alteration. It still only matters what you have. Are you a male producer or female reproducer. Are you functional or not. Do you have both sets? That’s basically it, sex just cares about the facts it’s not socially constructed. You can’t argue that someone with a penis does not have a penis.

Ethnicity tho, doesn’t have a modern medicine equivalent. It just is what it is. You can’t change facts, so your rather stuck with it. Unlike a penis.