AFAB is “assigned female at birth”, it’s used sometimes when people are trying to describe their experience as a trans person or non binary person, AMAB is the male version.
And yes same I feel like I have so many different backgrounds for people as couples
I guess because I’m multi cultural, and many of my friends were in the city I grew up in, i see it very normal. But people are very unnormal about it else I’ve realized, so yes it is important to celebrate and acknowledge their heritages 
Theo is from an Iranian family and their mom is white but they’re closer connected to their Iranian side, the story takes places in the U.K. Kay’s grandma is white too and I think he probably has other European ancestry but the rest of his family is mostly Filipino. Both their families are from immigrant families!