I know a bit about the setting, but that's 2/3rd ed stuff.
No Tieflings then.
Then I started playing BG3 EA ... I'm confused ...
As in the DnD movie Thieve's Honor the ZOMG! OMG!WTF 2 CUTE!!!11 Doric is a - to use Warhammer termingology - a Gave, i.e. a Beastling ... erm ... Tiefling born to Human parents, a mutant, I thought that the Bg3 Tieflings are some sort of fresh mutants too.
That Karlach was a human street kid from Baldur's Gate sold into soldier-slavery in Avernus where she was mutated (go Clan Moulder!) into a Devil Stormtrooper, that the refugees in Druidville are formerly Human inhabitants of Eturiel who mutated while in Avernus.
But playing as Karlach and actually talking with other Tieflings made me question that - I now suspect

that the refugees and Karlach are Tieflings born into existing, established Tiefling communities in Faerun.
Was my first conjecture correct, or the second?