How did medieval Europeans think about race (if at all)? Do any writings survive on the subject of racial differences (skin tone, et al) and what, if any, significance was attributed to these differences? Was there any medieval corollary to the “scientific racism” of the 19th & 20th centuries?

u/sunagainstgold dealt with the medieval issue several years ago, in a related question about why African people are called "black"

In addition to the sources she listed back then, there has been a lot of work done more recently, perhaps most importantly Geraldine Heng, The Invention of Race in the European Middle Ages (Cambridge University Press, 2018). I don't recall any AH answers dealing with Heng's book specifically but maybe someone else can talk about it here.