I used to try to match them precisely, but I think the continents, at least, are more or less matched to ours but exaggerated and simplified for comedic effect. XXX (or EcksEcksEcks) is Australia, the Counterweight Continent is Eastern Asia, anything Hubward seems to be Scandinavian/Russian. Rincewind once went somewhere very Aztecan/Mayan, but I don’t remember if he even knew where it was. Ephebe and Tsort are Greco-Roman, with Djelybeybi serving as the Egypt of Discworld.
I always assumed Ankh-Morpork was a combination of NYC and London (or just a stand-in for big cities everywhere), but UK dopers might have a better insight into that.
Oh, and Llamedos is vaguely Welsh. (Apologies if any of this is inaccurate characterization… I’ve only made it as far as Canada so far.)
I always wondered if Terry Pratchett lumped the UK/US/Canada into one thing because there weren’t enough recognizable differences between the three to make a parody recognizable or funny. Or did I miss a big joke?