How do people even know someone's race, religion, or gender?

So I started thinking about how diverse my family is. My own immediate family looks so different even though we are all Spaniard-Cuban predominantly. My mother's side came from Barcelona and so she's very pale with freckles and looks almost German. My father's side came from Northern Spain and he looks Arab because of the Moors. My sister looks Chinese because of our great grandfather who was Chinese-Cuban. And I look French because of again the Northern Spain ancestory. So it just makes me think how do people ever assume race? And on top of that I've had friends who are gender fluid so it's like how can people assume gender? Another thing is how does one assume someone's religion? Like obviously if someone is wearing a hijab or a yamaka it's kind of obvious but I know plenty of Muslim women who don't wear anything and plenty of Jewish guys who don't either. So it's like how does anyone assume? Has anyone else ever wondered that?